fix(pi): resume at requested cwd when the session's directory was removed

[fgrehm]

Human comments

Similar to the other PR I just opened, this almost drove me nuts 😅 I'm now rebuilding my container with both patches and will report back if anything goes wrong.

What was wrong

When update_environment_directory moved a thread to a new environment (e.g. the agent deleted the old worktree first), the next turn died at the bridge: thread/resume rejected with Cannot resume: the pi session's working directory "...no longer exists." because the pi session file still recorded the old cwd — even though the runtime supplied a valid, existing requested cwd. In the observed case the thread was effectively bricked until an external stop/recovery.

Root cause: resumedSessionMissingCwd (plugins/provider-pi/src/bridge/bridge.ts:532) rejects the whole resume whenever the persisted cwd is gone, without considering the requested cwd.

What changed

• thread/resume now only rejects when the requested cwd is also missing — i.e. there is genuinely nowhere to run. With the persisted cwd gone but a valid requested cwd present, resume proceeds.
• constructPiThreadSession prefers the persisted session cwd only when the directory still exists (new usablePersistedSessionCwd()), falling back to the requested cwd for the child spawn and delta-translation sessionCwd. When the persisted cwd still exists, behavior is unchanged (session reports the header cwd).
• No wire/protocol changes; provider-bridge-only. No CLI/guide/doc updates needed.

How you verified

• New regression test resumes at bb's requested cwd when the session header's cwd was removed (bridge.round2.test.ts): red before the fix (reproduced the exact user error text), green after.
• Existing a resumed thread reports the session header's cwd, not the cwd bb asked for still passes — persisted-cwd precedence preserved when the dir exists.
• bridge.round2 + bridge.settings + bridge.checkpoint-fork: 15/15 pass. turbo run typecheck --filter=bb-plugin-provider-pi: clean.

AGENT GENERATED
_{Generated with "Omen Alpha" (Opencode Go) via BB / Pi}