fix(grep): recognize -D/--devices as a value-taking flag

[gene1wood]

Summary

GNU grep's -D ACTION / --devices ACTION (read|skip, how to handle
device/FIFO/socket files) takes a value, exactly like -d/--directories.
RTK's VALUE_FLAGS_SHORT recognized lowercase -d but not uppercase -D,
and VALUE_FLAGS_LONG had no --devices entry at all. As a result the
action word (skip) was left as an ordinary positional, which shifted the
real search pattern and path out of position — the shifted "path" usually
doesn't exist, so grep exited 2 with no output instead of searching.

grep     -D skip apple f1.txt   # 3 matches, exit 0
rtk grep -D skip apple f1.txt   # (empty), exit 2 — before this fix

Fix

• Add D to VALUE_FLAGS_SHORT in src/cmds/system/search.rs, alongside
  its lowercase -d twin.
• Add --devices to VALUE_FLAGS_LONG so the space-separated long form
  (--devices skip) also consumes its value correctly. (--devices=skip,
  a single token, already worked.)

Test plan

• Added unit tests in search.rs covering -D skip, inline -Dskip,
  --devices skip, and --devices=skip.
• Added an end-to-end byte-for-byte test in
  tests/grep_faithful_format_test.rs (dash_capital_d_devices_matches_grep_n)
  that runs the compiled rtk binary against real grep -D/--devices and
  diffs stdout + exit code, with and without -n — the same harness shape
  that would have caught this as a regression.
• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk <command> output inspected

Fixes #3881

[CLAassistant]

All committers have signed the CLA.

[gene1wood]

Closing in favor of #3681 which should fix this.