feat(writes): enable write actions by default (opt-out)

Invert the global write toggle so writes are ENABLED by default and the
user opts OUT, while keeping all other safety intact (per-call confirm=true
dry-run preview and the on-disk audit log are unchanged).

- writes.py: use a `writes_disabled` flag file instead of `writes_enabled`;
  `writes_enabled()` returns True unless the flag exists (env var still
  honored either way); `set_writes(False)` creates the flag, `set_writes(True)`
  removes it. Update user-facing notes.
- tests: default is now ENABLED; adjust toggle/disabled tests (set disabled
  explicitly via set_writes(False) or FAMILIAR_ENABLE_WRITES=0).
- README.md / CLAUDE.md: document writes are on by default, opt out via
  `familiar writes disable`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -19,7 +19,7 @@ uv run ruff check .           # lint (run both before committing; CI enforces th
 uv run familiar               # run the MCP server over stdio (how Claude launches it)
 uv run familiar login         # browser EVE SSO login (adds a character; run once per character)
 uv run familiar status        # list logged-in characters
-uv run familiar writes enable # allow write actions (default: disabled)
+uv run familiar writes disable # block write actions (default: enabled)
 ```
 
 There is no separate build step; `uv build` produces the wheel (CI does this on merge).
@@ -34,7 +34,7 @@ There is no separate build step; `uv build` produces the wheel (CI does this on
 
 **Per-character / per-corp pattern.** [charutil.py](src/familiar/charutil.py) is the backbone of every authenticated tool. `run_per_character(character, fetch)` runs a fetcher for one named character or **all** logged-in ones (when `character` is None), returning a uniform `{"characters": [...]}`. `run_per_corp` additionally resolves each character's `corporation_id` (corp endpoints need the right corp role or ESI 403s). `one_token` picks a single token for thing-scoped lookups (e.g. a structure). `summarize(items, limit)` gives `{count, sample}`. account.py and all read modules build on these.
 
-**Write safety — the choke point.** [writes.py](src/familiar/writes.py) `guard(action, params, confirm, execute)` is the single gate every mutating tool calls. `confirm=False` → dry-run preview (nothing happens); `confirm=True` but writes disabled → `WriteDisabled`; `confirm=True` + enabled → execute + append to an audit log. Writes are **off by default** (toggle via `familiar writes enable` / `FAMILIAR_ENABLE_WRITES`). Write tools take a *required* `character` and a `confirm` flag; reads take an *optional* `character`. When adding a write tool, route it through `writes.guard` — never call a mutating ESI endpoint directly.
+**Write safety — the choke point.** [writes.py](src/familiar/writes.py) `guard(action, params, confirm, execute)` is the single gate every mutating tool calls. `confirm=False` → dry-run preview (nothing happens); `confirm=True` but writes disabled → `WriteDisabled`; `confirm=True` + enabled → execute + append to an audit log. Writes are **on by default** (opt out via `familiar writes disable`; toggle via `familiar writes enable`/`disable` or `FAMILIAR_ENABLE_WRITES`). Write tools take a *required* `character` and a `confirm` flag; reads take an *optional* `character`. When adding a write tool, route it through `writes.guard` — never call a mutating ESI endpoint directly.
 
 ## Testing conventions