feat(admin): local-only stdio MCP server to mint scoped service-account tokens (dispatch#67)

dispatch#67 fix #2 needed a read-only CI credential and hit a hard wall:
Gitea's own token-creation and admin-user endpoints reject token auth
outright -- POST /users/{u}/tokens returns 'auth required', POST
/admin/users returns 'token does not have at least one of required
scope(s), required=[write:admin]' -- for ANY existing token, by design. An
existing token cannot mint another token or create an account; the only
way through either endpoint is real basic auth as an actual site admin.
That is exactly what a sandboxed agent session cannot safely hold.

internal/adminbootstrap: the core bootstrap, HTTP-only, fully testable
against a fake transport (5 tests, no network). Given real admin
username/password, it creates the target account (or resets its password
if it already exists -- deliberately unconditional, gated behind a
required ConfirmServiceAccount flag since that reset is only safe for a
dedicated bot account), mints it a scoped token by authenticating AS that
account (Gitea requires this specifically -- an admin cannot mint on
another account's behalf), grants it read-only collaborator access on
named repos, and stores the result via a SecretStore -- never returning
the token or either password to the caller.

internal/localtools: the MCP tool wrapper (mint_service_account_token) and
the real op-CLI-backed SecretStore (token passed via stdin, never argv,
per the estate's own secret-handling rule). Deliberately its OWN package,
not internal/tools -- this capability is far too dangerous to risk it ever
being wired into RegisterAll and reachable from the shared HTTP gitea-mcp
server, even behind OIDC.

internal/mcp/stdio.go: a stdio JSON-RPC transport (one message per line,
standard MCP stdio framing) for the existing registry.Tool interface --
the only transport gitea-mcp had before this was HTTP (server.go), which a
sandboxed session can reach but a real admin password never should be
piped through.

cmd/gitea-admin-mcp: the binary. Meant to be spawned by a LOCAL Claude Code
session (.mcp.json stdio server) or run by hand, via 'op run --env-file
~/.op-env' so admin credentials arrive just-in-time and never touch a
sandboxed session.

11 new tests total, all passing under -race. Nothing in cmd/gitea-mcp (the
shared HTTP server) changed.

internal/localtools/mint_service_account_token_test.go

@@ -0,0 +1,110 @@
+package localtools_test
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+
+	"git.d-ma.be/mathias/gitea-mcp/internal/adminbootstrap"
+	"git.d-ma.be/mathias/gitea-mcp/internal/localtools"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// This tool lives in its own package, never internal/tools, on purpose: it
+// resets whatever password its target account has, which is far too
+// dangerous a capability to risk it ever being wired into RegisterAll and
+// exposed on the shared HTTP gitea-mcp server. Only cmd/gitea-admin-mcp (a
+// separate, local-only, stdio binary) imports this package.
+
+type fakeBootstrapper struct {
+	gotReq adminbootstrap.Request
+	result adminbootstrap.Result
+	err    error
+}
+
+func (f *fakeBootstrapper) Run(_ context.Context, req adminbootstrap.Request) (adminbootstrap.Result, error) {
+	f.gotReq = req
+	return f.result, f.err
+}
+
+func TestMintServiceAccountToken_ParsesArgsAndCallsBootstrapper(t *testing.T) {
+	fb := &fakeBootstrapper{result: adminbootstrap.Result{
+		Username: "dispatch-bot", Created: true,
+		SecretRef: "op://AgentSecrets/gitea-dispatch-bot-ci-readonly/token",
+	}}
+	tool := localtools.NewMintServiceAccountToken(fb)
+
+	args, _ := json.Marshal(map[string]any{
+		"username":                "dispatch-bot",
+		"email":                   "dispatch@d-ma.be",
+		"scopes":                  []string{"read:repository", "read:issue"},
+		"token_name":              "dispatch-ci-readonly",
+		"secret_name":             "gitea-dispatch-bot-ci-readonly",
+		"grant_read_repos":        []string{"mathias/cobalt-dingo"},
+		"confirm_service_account": true,
+	})
+
+	out, err := tool.Call(context.Background(), args)
+
+	require.NoError(t, err)
+	assert.Equal(t, "dispatch-bot", fb.gotReq.Username)
+	assert.Equal(t, []string{"read:repository", "read:issue"}, fb.gotReq.Scopes)
+	assert.Equal(t, true, fb.gotReq.ConfirmServiceAccount)
+	require.Equal(t, []adminbootstrap.RepoRef{{Owner: "mathias", Name: "cobalt-dingo"}}, fb.gotReq.GrantReadRepos)
+
+	var result map[string]any
+	require.NoError(t, json.Unmarshal(out, &result))
+	assert.Equal(t, "op://AgentSecrets/gitea-dispatch-bot-ci-readonly/token", result["SecretRef"])
+	// The tool's own output must never carry a raw token or password field --
+	// adminbootstrap.Result structurally can't (it has none), which this pins.
+	_, hasToken := result["Token"]
+	_, hasPassword := result["Password"]
+	assert.False(t, hasToken)
+	assert.False(t, hasPassword)
+}
+
+func TestMintServiceAccountToken_RejectsMalformedRepoRef(t *testing.T) {
+	fb := &fakeBootstrapper{}
+	tool := localtools.NewMintServiceAccountToken(fb)
+
+	args, _ := json.Marshal(map[string]any{
+		"username":                "dispatch-bot",
+		"email":                   "dispatch@d-ma.be",
+		"scopes":                  []string{"read:repository"},
+		"token_name":              "x",
+		"secret_name":             "x",
+		"grant_read_repos":        []string{"not-owner-slash-repo"},
+		"confirm_service_account": true,
+	})
+
+	_, err := tool.Call(context.Background(), args)
+
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "owner/repo")
+}
+
+func TestMintServiceAccountToken_PropagatesBootstrapperError(t *testing.T) {
+	fb := &fakeBootstrapper{err: assert.AnError}
+	tool := localtools.NewMintServiceAccountToken(fb)
+
+	args, _ := json.Marshal(map[string]any{
+		"username":                "dispatch-bot",
+		"email":                   "dispatch@d-ma.be",
+		"scopes":                  []string{"read:repository"},
+		"token_name":              "x",
+		"secret_name":             "x",
+		"confirm_service_account": true,
+	})
+
+	_, err := tool.Call(context.Background(), args)
+
+	require.Error(t, err)
+}
+
+func TestMintServiceAccountToken_Descriptor(t *testing.T) {
+	tool := localtools.NewMintServiceAccountToken(&fakeBootstrapper{})
+	d := tool.Descriptor()
+	assert.Equal(t, "mint_service_account_token", d.Name)
+	assert.NotEmpty(t, d.Description)
+}