docs: REUSE.md introduced — cross-repo reuse map (exec/format-safety guards, durable-id seams, traps; docs-only, no version bump)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

CLAUDE.md

@@ -50,6 +50,7 @@ internal/pbs/        PBS-API client (fingerprint-pinned) + verify maintenance lo
 
 - Design doc: `felhom.eu/documentation/architecture/03-host-agent.md` (locked).
 - Platform facts: `felhom.eu/documentation/proxmox-platform.md` + `tests/phase{0,1-2,3,4}-findings.md`.
+- See `REUSE.md` before writing new code.
 
 ## Current state
 
@@ -95,6 +96,7 @@ All shippable work commits **directly to `main`**; `main` is always equal to wha
 > **Never write secrets** — tokens, passwords, private keys, API keys — into `CHANGELOG.md`, `REPORT.md`, or any committed file. Reference them as "stored out-of-band" instead.
 
 - Code quality: verify generated code for bugs/edge cases; add debug logging; **ask rather than guess** when you'd otherwise invent input/output.
+- Update `REUSE.md` if you added/changed/deprecated a shared helper or pattern (same commit).
 
 ### Live validation