fix(prompt): freeze static system prompt per session to stop mid-session KV-cache flushes

Problem

The static system prompt is re-read from disk on every API call. When the file changes mid-session (e.g. an external writer appends to a global AGENTS.md), the entire static prompt changes, which invalidates the provider KV/prefix cache and forces a full ~160k token re-upload on every call until the writer stops. This makes sessions unusable under concurrent writers.

Fix (ready to merge)

Branch fix/static-prompt-freeze on fork GroepOnline/jcode (rebased on current master): https://github.com/GroepOnline/jcode/tree/fix/static-prompt-freeze

Diff: https://github.com/1jehuang/jcode/compare/master...GroepOnline:fix/static-prompt-freeze.diff

Three commits:

• cc051d5de fix(prompt): freeze static system prompt per session (KV-cache) — introduces StaticPromptLock, frozen once per session (same philosophy as locked tools); busts only on skills-fingerprint / canary / working-dir change. Dynamic parts still update per call via append_dynamic_prompt_parts.
• d26551714 fix: Bust frozen prompts on config reload
• 16f081167 fix(tests): PromptLockProvider stub mist Provider::fork

Verification

• app-core regression tests 3/3 green (incl. new test asserting the prompt stays stable when the underlying file mutates mid-session)
• clippy 0 warnings, tui check ok
• Fork PR #1 in GroepOnline/jcode was reviewed and merged there first

Note

Fork PRs are not accepted on this repo, so please cherry-pick from the branch above or open a branch here. Happy to adjust based on maintainer preference.

Start by reviewing the fix/static-prompt-freeze branch and the three listed commits, especially the StaticPromptLock changes and app-core regression tests. Run the reported 3/3 app-core tests and verify that the static prompt remains stable when its underlying file changes during a session, while dynamic prompt updates and config-reload busting still behave as described.