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>

README.md

@@ -8,7 +8,7 @@ Ask your familiar where your assets are, what your skill queue looks like, what
 
 ## Principles
 
-- **Safe by default.** Reading is free; every *write* — sending mail, setting waypoints, editing contacts/fittings/PI, fleet commands — is off until you turn it on, previewed before it runs, and recorded in an audit log. The AI can see everything and changes nothing unless you let it.
+- **Safe by default.** Reading is free; every *write* — sending mail, setting waypoints, editing contacts/fittings/PI, fleet commands — is previewed before it runs (nothing happens without `confirm=true`) and recorded in an audit log. Writes are on by default but can be turned off entirely with `familiar writes disable`. The AI can see everything and changes nothing without your explicit confirm.
 - **Local-first.** Runs on your own machine. Your refresh token and account data never leave your control.
 - **Everything, eventually.** The goal is full coverage: the ESI scopes you grant (character *and* corp, read and write), plus the complete static universe. Built in phases so it actually ships.
 
@@ -49,12 +49,14 @@ Claude Desktop (`claude_desktop_config.json`):
 
 Then ask in natural language — "what's Tritanium worth in Jita?", "how much ISK across all my characters?", "where are my characters?".
 
-### Enabling writes
+### Write actions
 
-Write actions (send mail, set waypoints, edit contacts/fittings, fleet commands) are **off by default**. Every write tool also returns a dry-run preview unless called with `confirm=true`, and each executed write is recorded in an audit log.
+Write actions (send mail, set waypoints, edit contacts/fittings, fleet commands) are **on by default**. Even so, every write tool returns a dry-run preview unless called with `confirm=true` per call, and each executed write is recorded in an audit log — so nothing changes by accident.
+
+To opt out and block all writes globally:
 
 ```bash
-uv run familiar writes enable   # allow writes (uv run familiar writes disable to revoke)
+uv run familiar writes disable   # turn writes off (uv run familiar writes enable to re-allow)
 ```
 
 ## Roadmap