fix(framework): install deps on worktree bootstrap + legible deps-preflight at gate seam (#856)

Worker git-worktrees under a fresh `git worktree add` have empty
node_modules/.bin (pnpm workspaces do not share node_modules across
worktrees), so every gate binary (tsc/eslint/prettier/vitest) fails with
a raw `sh: 1: <tool>: not found` before the QA gate suite ever runs a
real check. That failure is indistinguishable from a genuine test/lint
failure -- a false-red gate that was responsible for 131/147 fleet
tool-errors.

- plugins/mosaic-framework/src/index.ts: the injected "Git Worktree
  Requirement" instructions now direct every worker to run
  `pnpm install --frozen-lockfile --prefer-offline` immediately after
  `git worktree add`/`cd`, before any gate is invoked.
- packages/mosaic/framework/tools/qa/qa-hook-stdin.sh: the common
  gate-entry seam (registered as the PostToolUse hook for every
  Edit/Write/MultiEdit in runtime/claude/settings.json) now preflights
  node_modules/.bin presence for JS/TS files and fails with an explicit
  "deps not installed -- run pnpm install" sentinel instead of silently
  letting a raw not-found surface downstream.
- packages/mosaic/framework/tools/qa/test-deps-preflight.sh: new
  regression harness (wired into test:framework-shell) covering missing
  node_modules/.bin, empty node_modules/.bin, populated node_modules/.bin,
  and non-JS/TS skip behavior.

Closes #856.

plugins/mosaic-framework/src/index.ts

@@ -217,12 +217,22 @@ git fetch origin
 mkdir -p ~/src/${projectName}-worktrees
 git worktree add ~/src/${projectName}-worktrees/<task-slug> -b <branch-name> origin/main
 cd ~/src/${projectName}-worktrees/<task-slug>
+pnpm install --frozen-lockfile --prefer-offline
 # ... all work happens here ...
 git push origin <branch-name>
 cd ~/src/${projectName} && git worktree remove ~/src/${projectName}-worktrees/<task-slug>
 \`\`\`
 
-Worktrees path: \`~/src/<repo>-worktrees/<task-slug>\` — NEVER use /tmp.`);
+Worktrees path: \`~/src/<repo>-worktrees/<task-slug>\` — NEVER use /tmp.
+
+\`pnpm install --frozen-lockfile --prefer-offline\` MUST run immediately after
+\`git worktree add\`/\`cd\`, BEFORE any gate (\`pnpm test\`/\`lint\`/\`typecheck\`/\`format:check\`)
+is invoked. pnpm workspaces do NOT share \`node_modules\` across separate git
+worktrees — a fresh worktree has an empty \`node_modules/.bin\`, so every gate
+binary (\`tsc\`/\`eslint\`/\`prettier\`/\`vitest\`) fails \`sh: 1: <tool>: not found\`
+until deps are installed. That failure is indistinguishable from a real
+test/lint failure — a false-red gate. Never skip this step and never reorder
+it after the first gate invocation.`);
 
   // 6. Completion gates
   sections.push(`# Completion Gates — ENFORCED