fix(agent-connector): run opencode in the configured working directory (#435)

[nolanchic]

Fixes #435

agn create <name> --type opencode --path <project> ignored the configured path: the opencode adapter documented opts.workingDir but never used it, hardcoding --dir and cwd to the per-agent scratch dir (~/.openagents/agents/<name>).

opencode confines an agent's file access to its --dir worktree root, so in headless mode any read/edit tool call against a real project file is auto-rejected with "The user rejected permission to use this specific tool call." — on every platform, not only Windows.

Fix

• Run opencode in the configured workingDir (both --dir and the spawn cwd) via a new _resolveCwd() helper. When no path is configured it falls back to agentHome, so pre-existing agents keep running exactly where they always did.
• Scope the persisted session store per working directory (sessions-<sha256[:16]>.json). An opencode session ID is only resumable against the --dir it was created under, so without scoping a path change (or an agent that previously ran in agentHome getting a path configured) would poison the next run with a stale ID that 404s. The default (no workingDir) keeps the legacy sessions.json so existing agents retain their context. Files always live under agentHome, never inside the project tree. This mirrors the sha256[:16] per-cwd idiom already used by the goose and nanoclaw adapters.
• Preflight now access-checks the resolved working dir and surfaces cwd_unavailable (naming the configured path) when it is missing or unreadable, instead of silently running in agentHome.

this.workingDir is already populated by BaseAdapter and passed by daemon.js (workingDir: agentCfg.path || defaultAgentWorkdir(name)), so no daemon changes are needed — the adapter just had to use it.

Test plan

• node --test test/opencode-workingdir.test.js — 7/7 new tests pass. Covers _resolveCwd (configured vs fallback), --dir/cwd passed to the real subprocess (asserted via a child_process.spawn shim installed before require, same pattern as test/aider.test.js), per-working-dir session-store scoping, and the preflight cwd_unavailable/pass cases.
• node --test test/opencode.test.js — existing 8.1–8.5 suites pass. (Two preflight credential tests are non-hermetic — they inspect the real home dir and fail when a real ~/.config/opencode/opencode.json exists on the machine; identical failures occur on develop, unrelated to this change.)
• Manual: agn create demo --type opencode --path /path/to/project; connect to a workspace; @mention it to read or edit a project file and confirm the "user rejected permission" error is gone; send a second message in the same channel and confirm context is retained.

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