fix(grep): honor -c count flag (#1452)

[ousamabenyounes]

Problem (#1452)

rtk grep -c PATTERN PATH ignored the -c flag and returned the regular match summary + context lines instead of a line count. Plain grep -c and rg --count emit a single integer (or path:N per file) — the existing rtk behaviour broke any script doing n=$(rtk grep -c …).

Root cause

The Grep subcommand has context_only: bool whose short alias is -c. Clap consumed the bare -c token before the new count flag could be parsed, silently flipping context_only on and dropping the pattern positional.

Fix

1. New --count flag on Grep (src/main.rs) routed to a dedicated run_count path in grep_cmd.rs.
2. run_count helper invokes rg --count (with a GNU grep -c fallback) and mirrors plain grep -c semantics: bare integer for a single regular file, path:N per file for directory / multi-file. Tracking still records the command.
3. preprocess_argv argv rewrite (src/main.rs) translates the literal -c token into --count only when the grep subcommand is in play. Other subcommands (e.g. git -c user.name=…) are untouched. This is the minimal escape hatch for the short-alias collision; no other rtk subcommand currently needs it.

Test plan

• cargo fmt --all
• cargo test --all — 1683 passed, 0 failed
• New regression tests in src/main.rs:
  • test_preprocess_argv_grep_dash_c_rewritten — rtk grep -c TODO src/ parses with count=true, context_only=false
  • test_preprocess_argv_non_grep_dash_c_untouched — rtk git -c user.name=Foo log is left alone
• Smoke test against release binary:
  • rtk grep -c foo /tmp/t.txt → 3 (matches plain grep -c)
  • rtk grep -c TODO src/cmds/system/ → path:N per file (matches grep -rc)

Closes #1452

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