refactor(git,gists): type repository tree and gist MCP tools

[SamMorrowDrums]

Summary

Next slice of stack #3385: concrete typed outputs for get_repository_tree, list_gists, get_gist, create_gist, update_gist. Stacked on #3398.

Why

Part of #3385.

What changed

• Typed outputs + explicit output schemas (gist schemas drop null from omitempty fields; tree size is optional integer).
• Inputs stay map-based; per-tool normalizers run legacy validators in legacy order with identical messages and canonicalize page/perPage.
• Legacy/unknown protocol clients still get no schema/structuredContent (shared gate).
• New wire/schema tests across 2026-07-28, 2025-11-25, empty and unknown protocols; *_typed snapshots.

MCP impact

• Tool schema or behavior changed — output schema/structuredContent on 2026-07-28 only; text payloads unchanged.

Prompts tested (tool changes only)

• Wire tests: tree with recursive/path_filter, list gists with string pagination/since, get/create/update gist.

Security / limits

• Auth / permissions considered — scopes and IFC labels unchanged.

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint (0 issues)
• Tested locally with ./script/test (pass)
• UPDATE_TOOLSNAPS=true go test ./... && script/lint && script/test && script/generate-docs && git diff --check all exit 0.

Docs

• Not needed (generate-docs produced no changes)