feat: several Claude subscriptions, bound per session

[pocharlies]

One OpenCode server drives several Claude subscriptions at once, with each
session pinned to one — this chat on work, that one on personal.

Each account is a CLAUDE_CONFIG_DIR. That shape is forced by the rotation
constraint this codebase already documents in auth-login.ts: Anthropic rotates
the refresh token on every use, and a chain with two owners gets the whole grant
revoked for replay. Giving each account its own CLI home keeps exactly one owner
per chain — the CLI — so accounts cannot race each other's rotation. The plugin
reads those credentials and never rotates them.

Selection rides the model id. opus@personal, shown as Opus 5 · Personal,
flows through the existing chat.headers → EFFORT_HEADER path, so it appears
in the host's picker with no UI work. The first turn binds the session; later
turns stay on that account even when the request carries none.

Everything previously global is now keyed by account:

• rate-limit state and the 429 fast-fail gate — an exhausted subscription
  used to block every other account, which defeats having them. Existing
  single-account stores migrate on read.
• session bindings, and resume lookups against the owning account's
  transcript dir. Moving accounts drops the stale resume target rather than
  continuing a foreign conversation in another home.
• the credential probe and credential reads, which now refuse to fall back
  to the ambient ~/.claude for a scoped account — that would silently run the
  turn on the wrong subscription.

Declaring no accounts changes nothing: same model ids, same stores, same
behaviour. Configure with the accounts plugin option,
OPENCODE_CLAUDE_ACCOUNTS, or accounts.json.

One caveat worth stating: CLAUDE_CONFIG_DIR also relocates settings, skills
and the user-level CLAUDE.md, so shared files need symlinking into each home.
Documented in the README.

Also isolates XDG_DATA_HOME for the test run — the suite was reading the
operator's real accounts.json (so "nothing configured" stopped being true once
they had accounts), writing fixture sessions into their store, and unlinking
their debug.log.

bun test passes and tsc is clean. Independent of #9.