fix(skill+gemini): forgive Skill schema confusion + sanitize JSON-Schema for Gemini

[setterapp]

This PR fixes two model-side / provider-side bugs surfaced when running v0.12.0 against MCP servers (notion, supabase) on macOS.

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Fix 1: skill_manage schema confusion (commit f477bd4)

Models invoking jcode produce malformed calls like skill_manage(action="load") (missing name) and error with 'name' is required for load action. Root cause: the public-facing tool is Skill with schema {skill}, internal tool is skill_manage with schema {action, name}. The translation layer in src/tool/mod.rs was half-applied at v0.12.0 (mapping draft + transform_input were committed but the json import was missing — the file did not compile).

This PR:

1. Adds the missing json import to src/tool/mod.rs.
2. Keeps the Skill | skill -> skill_manage resolve and {skill: X} -> {action: load, name: X} transform.
3. Adds #[serde(alias = "skill")] to SkillInput.name so skill_manage called directly with {action: load, skill: brain} also works.
4. Improves the error message when name is genuinely missing.

Fix 2: Gemini rejects MCP tool schemas (commit on same branch)

Gemini's generateContent endpoint uses an OpenAPI 3.0 schema subset and rejects requests with HTTP 400 when tool parameters contain $defs, $ref, or $schema — which is exactly what most MCP servers emit (notion, supabase, etc.) because they generate schemas from typed code (Pydantic / Zod) that produces $defs+$ref.

Repro: configure notion or supabase MCP, then jcode --provider gemini run "hi" — fails with hundreds of errors like:

Unknown name "$defs" at 'request.tools[0].function_declarations[17].parameters'
Unknown name "$ref" at 'request.tools[0].function_declarations[18]...'
Unknown name "$schema" at 'request.tools[0].function_declarations[40]...'

This PR extends gemini_compatible_schema in src/provider/gemini.rs to:

1. Extract $defs (or definitions) from the root.
2. Recursively walk the schema, resolving $ref to the corresponding $def and inlining the target.
3. Strip JSON-Schema metadata ($defs, $schema, $id, $comment, $anchor, definitions, title) at every level.
4. Cap recursion depth at 24 to handle circular schemas safely.

Preserves the existing const → enum rewrite.

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Tested

• cargo check -p jcode --bin jcode — passes
• cargo build --release -p jcode --bin jcode — passes (4m24s first build, 1m08s incremental)
• Smoke 1: Skill tool loads jcode skill end-to-end with Claude Sonnet OAuth in TUI.
• Smoke 2: jcode --provider gemini --model gemini-2.5-flash run "Reply only OK" from setterapp dir (notion + supabase-dev + supabase-prod MCPs configured) — works after fix, was failing with HTTP 400 before.

Diff stat

 src/tool/mod.rs       | 28 ++++++++++++++++++++++++++++-
 src/tool/skill.rs     | 13 +++++++++++--
 src/provider/gemini.rs | 64 +++++++++++++++++++++++++++++++++++++--
 3 files changed, 100 insertions(+), 5 deletions(-)

No public API changes.