feat(workplace): per-project custom working directory

[CatJuly]

Summary

Every project's workplace is currently pinned to ../OpenOPC_workplace/<project_id>/ beside the OpenOPC repo, with no way to relocate it. In practice most users want agents to work inside an existing repository (their actual codebase), not in a freshly created empty folder. Today the only workarounds are symlinks or spelling out the path in every prompt.

This PR makes the workplace configurable per project, end to end: the path a user sets is the cwd the execution agent (claude_code / codex / cursor / opencode) is spawned with.

Design

Resolution is three-tiered, most specific first:

1. Per-project path — stored in a new project_config table inside that project's own tasks.db
2. $OPC_WORKPLACE_ROOT — global override; each project resolves to $OPC_WORKPLACE_ROOT/<project_id>
3. Default — the existing ../OpenOPC_workplace/<project_id>/ layout, unchanged

get_project_workplace() gains an optional custom_path parameter; callers without DB access (tests, preflight) keep the old behaviour untouched. The DDL is idempotent and runs on every store initialize(), so existing databases gain the table with no migration step. The engine loads the custom path into a synchronous cache during initialize(), keeping the existing sync resolvers' signatures unchanged.

Sticky session workspace roots that merely equal the project workplace are no longer persisted into execution_defaults — they carry no session-specific intent and previously froze the session to whatever the workplace was at first message, so later set-workplace changes never took effect. Sessions pinned before this rule self-heal on their next message. Roots derived from an explicit output dir keep sticking exactly as before (covered by tests/test_engine_session_defaults.py).

Surfaces

Office UI

• Create dialog: + opens a centered modal (same conventions as the delete-confirm / org-create dialogs) with project name and an optional workplace path. The name field validates live and previews the derived slug.
• Settings: a gear button next to the project selector shows the resolved path and its source tier (custom / env / default), pre-fills custom paths for editing, and offers reset-to-default.
• All dialogs use theme tokens, so the paper theme stays readable.

CLI

opc project set-workplace myapp --path /path/to/existing/repo [--create]
opc project show-workplace myapp
opc project reset-workplace myapp

Docs: README / README.zh-CN document the resolution order and commands.

Testing

• End to end: created a project pointed at an existing repository and ran a claude_code task — the agent reports cwd inside that repository (verified against a real session, including one that had been pinned to the old default and self-healed).
• set-workplace → show-workplace → reset-workplace round-trips through the project DB; a legacy DB (pre-existing tables, no project_config) upgrades in place with data intact.
• pytest failure set is identical to current main baseline (no new failures); tests/test_engine_session_defaults.py passes 7/7.
• Frontend npm run typecheck and npm run build clean; rebuilt frontend_dist included.

Stacked on #31

This branch is based on #31 (fix/project-create-name-feedback) because both touch ProjectSelector.tsx — the create-dialog rewrite here supersedes the inline validation added there. Until #31 merges, its commit (d7cfb2b) appears in this PR's diff; once #31 lands, this PR reduces to its own two commits. Merging #31 first is recommended.