fix(qoder): honor QODER_CONFIG_DIR for mode state

[Vaishnavi220506]

Summary

Honor Qoder's QODER_CONFIG_DIR when Ponytail stores, reads, and removes its mode flag. This keeps mode switching and subagent injection in Qoder's selected user directory and lets uninstall clean the same flag.

Problem and cause

With QODER_SESSION_ID and QODER_CONFIG_DIR=<custom> set, /ponytail ultra writes ~/.qoder/.ponytail-active; <custom>/.ponytail-active is absent. The runtime and uninstaller both hardcode ~/.qoder even though Qoder supports the directory override. An isolated probe reproduced this on main at 552acd5. Fixes #1056.

Changes

• Resolve the Qoder directory once, using QODER_CONFIG_DIR with the current ~/.qoder fallback.
• Use it in the hooks and uninstall script.
• Cover mode switching, subagent readback, isolation from the default directory, and cleanup of a custom directory.

Testing

• New isolated hook probe on base: failed (<custom> flag missing; default flag written). New uninstall assertion on base: failed (custom flag remained).
• npm test on this HEAD (764720f) under Ubuntu/Node 22 with the repository's documented pandas dependency: passed (127 root tests and 28 Pi tests).
• node tests/uninstall.test.js on Windows: passed.
• node scripts/check-rule-copies.js, node scripts/check-versions.js, git diff --check: passed.
• Windows tests/hooks.test.js stops at the pre-existing path assertion tracked by test: compare the statusline copy path JSON-escaped in hooks.test.js #1046 before reaching the new Qoder case; the Qoder hook tests passed under Ubuntu.

No ruleset text changed, so the three-arm ruleset benchmark is not applicable under CONTRIBUTING.

[6639835]

Reviewed Qoder resolution across config, runtime state and uninstall at 764720f. The shared resolver honors QODER_CONFIG_DIR and preserves the default; reads, writes and uninstall derive from that location. Tests cover custom mode/subagent tracking and default/custom uninstall isolation. No actionable issue found in this static scope. Node hook/uninstall suites were not run.