feat(gptel-tools): wire update_text_file as a local tool with tests

I rewrote `update_text_file.el` in pure Elisp.  The previous version
shelled out to sed for everything, had a stray quote terminator at EOF
(line 149) that broke loading, produced literal backslash-n where
actual newlines were expected, and prompted via `y-or-n-p` redundantly
with gptel's own `:confirm t` flag.

The five operations -- replace, append, prepend, insert-at-line,
delete-lines -- split into pure string transforms that test without
touching the disk.  The file-level wrapper validates the path, enforces
a 10MB size limit, takes a timestamped backup, and writes atomically.
No backup is created when the operation is a no-op.  Patterns are
literal substrings (not regex) so the model can't trip over
metacharacter quoting.

`tests/test-update-text-file.el` covers Normal / Boundary / Error per
operation plus the file-level wrapper.  48 tests green.  Added
`update_text_file` to `cj/gptel-local-tool-features` so gptel exposes
the tool after restart.

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