framework: make tool discoverability, workspace placement and model tiering mechanical (#1174)

docs/PRD.md

@@ -1538,6 +1538,59 @@ All work is **alpha** (< 0.1.0) until Jason approves 0.1.0 beta release.
 
 ---
 
+## Workspace placement guard hardening (#1174)
+
+### Problem and objective
+
+The Bash pre-tool guard must prevent Git checkouts and repository state from being placed under
+`$HOME` without refusing ordinary Git commands merely because a source, option value, branch name,
+or metadata mentions `$HOME`. A guard that over-blocks routine work is unsafe because operators
+will route around it.
+
+### Scope and requirements
+
+1. `WPG-REQ-01`: `git clone` and `git worktree add` placement SHALL be judged from their placement
+   operands, not from every HOME-shaped word in the command.
+2. `WPG-REQ-02`: Clone sources, references, templates, environment assignments, and non-placement
+   worktree metadata MAY resolve under HOME when all placement operands resolve elsewhere.
+3. `WPG-REQ-03`: Both attached and separate-value `--separate-git-dir` forms SHALL remain placement
+   operands and SHALL be refused when they resolve under HOME.
+4. `WPG-REQ-04`: Option classification SHALL account for Git's rule-generated boolean negations
+   without relying on an enumerable allowlist of flag spellings.
+5. `WPG-REQ-05`: Quote removal, escapes, shell command boundaries, redirections, and end-of-options
+   handling SHALL preserve existing fail-closed checkout coverage.
+6. `WPG-REQ-06`: Absolute placement aliases SHALL resolve shell-known HOME spellings, dot segments,
+   repeated separators, and existing symlink parents before the HOME boundary comparison.
+7. Relative targets whose effective path depends on the shell cwd are out of scope and tracked by
+   #1197.
+
+### Acceptance and verification
+
+1. Git's own option parser accepts each tested flag, including generated `--no-*` forms, while the
+   guard allows a HOME-valued source with an explicit safe destination.
+2. Equivalent clone and worktree fixtures cover rule-generated negations and remain discriminating
+   against the prior head where the defect existed.
+3. Real HOME destinations and both `--separate-git-dir` forms remain blocked, including placements
+   after shell command boundaries.
+4. The full hermetic guard suite, syntax/static checks, adversarial probes, independent review, and
+   terminal-green CI pass before merge.
+5. Any option-classification residual is documented with its deliberate failure direction.
+
+### Constraints, risks, and assumptions
+
+- Security and usability are co-equal: neither a placement bypass nor routine over-block is an
+  acceptable repair.
+- `ASSUMPTION:` The value-taking option surface exposed by the installed Git version is closed and
+  measurable through Git's own parser/help output; rationale: boolean flags are rule-generated,
+  while separate-value options have explicit grammar and must be classified as such.
+- Risk: a future Git release may add a new value-taking placement option. Mitigation: document the
+  chosen residual direction and pin every currently supported placement option in behavior tests.
+- Risk: a symlink can be replaced after pre-execution canonicalization. Mitigation: resolve every
+  existing parent physically and document the remaining inherent TOCTOU window; the worktree helper
+  remains the authoritative path-derivation mechanism, with atomic closure tracked by #1199.
+
+---
+
 ## Assumptions
 
 1. RESOLVED: **pgvector is sufficient** for semantic search at v0.1.0 scale (personal/family/team = thousands to low hundreds-of-thousands of vectors). `@mosaicstack/memory` defines a `VectorStore` interface with pgvector as the default adapter. The interface boundary makes Qdrant a drop-in migration if PG resource contention or scale demands it later. Zero additional infrastructure for v0.1.0. Rationale: Reduces ops burden; pgvector HNSW indexes are fast at this scale; interface abstraction costs almost nothing now.