feat(gptel-tools): wire git_status / git_log / git_diff as local tools

Three read-only git context tools so gptel can see what's changed
without me pasting `git status` / `git log` / `git diff` output into
every chat turn.  Builds the first batch from the ADOPT bucket in
`docs/design/gptel-tools-shortlist.org`.

Shape per tool:

- `gptel-tools/git_status.el` — `git status --short --branch` for a
  directory inside a git working tree under HOME.  Returns the
  porcelain output, or a "Clean working tree" marker when only the
  branch line is present.
- `gptel-tools/git_log.el` — `git log --oneline -nN` with an optional
  `--since` filter.  N defaults to 20, capped at 100; nil / non-
  integer / out-of-range N falls back to the default.
- `gptel-tools/git_diff.el` — `git diff [REF1 [REF2]] [-- FILE]`.
  Output capped at ~500KB so a runaway diff can't blow up context;
  truncation is reported inline.

Validation is uniform: path must resolve under HOME, must be a
directory, must be inside a git working tree (verified via
`git rev-parse --is-inside-work-tree`).  Color is disabled via
`-c color.ui=false` at the git level (`git status` doesn't accept
`--no-color` directly).

Tests run against real temp git repos created via `process-file`,
not mocked — there's nothing in gptel-tools/git_*.el that's
process-mockable in a meaningful way, and a real `git init` + a
couple of commits is cheaper than building a fake.  31 tests total:
7 for git_status, 11 for git_log, 13 for git_diff.

Wired into `cj/gptel-local-tool-features` so gptel exposes the
three tools on next restart.

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