fix(brain): context, brain_get and brain_read read through brainfs (#157, slice 2)

Option 1 of #157, slice 2. readExcerpt (brain_context), api.ReadNote
(brain_get / read-after-write) and notes.ReadScoped (brain_read) read
the Syncthing-shared brain by path. Under a symlink swap race each
leaked an outside file hundreds of thousands of times in 1.5s, including
brain_read, whose Lstat check was raced by the reopen-by-name; a FIFO
blocked os.ReadFile.

- All three read through brainfs (os.Root, regular files only,
  non-blocking open, type check on the handle).
- brainfs.ReadRegularMax checks a size limit on the opened handle;
  notes keeps its ErrTooLarge.
- New internal/brainfs/brainfstest: the shared swap-race harness
  (symlink and held-writer FIFO swappers, awaited before TempDir
  cleanup; AssertNoLeak / AssertNoHang) for this and later slices.

Tests red first: each reader under a symlink-swap race and a held-writer
FIFO race, plus static checks. Flipped: readExcerpt back to os.ReadFile
is red.

Refs #157

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/mcp/tools_context.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"os"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -181,8 +181,13 @@ func readExcerpt(brainDir, relPath string, maxLen int) string {
 	if relPath == "" {
 		return ""
 	}
-	full := filepath.Join(brainDir, filepath.FromSlash(relPath))
-	content, err := os.ReadFile(full)
+	// Through the brain root, regular files only (hyperguild#157).
+	bfs, err := brainfs.Open(brainDir)
+	if err != nil {
+		return ""
+	}
+	defer func() { _ = bfs.Close() }()
+	content, err := bfs.ReadRegular(filepath.ToSlash(relPath))
 	if err != nil {
 		return ""
 	}