feat(flutter): add Flutter command filters

[copperspeed]

Summary

• Adds rtk flutter test -- suppresses passing progress lines, short-circuits to [PASS] All tests passed on clean runs, shows failure blocks with assertion details (~90% token reduction)
• Adds rtk flutter analyze -- keeps only warning - and error - lines, suppresses info - lines and headers (~80% token reduction)
• Adds rtk flutter build -- keeps only the final result line or error diagnostics (~85% token reduction)
• Adds rtk flutter pub -- strips download noise, keeps Got dependencies! summary and compatibility warnings (~70% token reduction)

Before / After

flutter test (passing, 6 lines -> 1 line):

# Before
00:00 +0: loading D:/dev/project/test/file_test.dart
00:01 +1: TsIconSize - constants compact is 20
00:01 +1: TsIconSize - constants standard is 24
00:02 +20: All tests passed!

# After
[PASS] All tests passed

flutter analyze (5 lines -> 2 lines):

# Before
Analyzing tradesmen...
warning - Dead code - lib/features/auth/...:60:50 - dead_code
warning - The left operand can't be null - lib/features/auth/...:60:53 - dead_null_aware_expression
   info - Unnecessary use of multiple underscores - test/...:113:11 - unnecessary_underscores

# After
warning - Dead code - lib/features/auth/...:60:50 - dead_code
warning - The left operand can't be null - lib/features/auth/...:60:53 - dead_null_aware_expression

flutter pub get (8 lines -> 2 lines):

# Before
Resolving dependencies...
Downloading packages...
  _fe_analyzer_shared 85.0.0 (98.0.0 available)
  flutter_riverpod 2.6.1 (3.3.1 available)
  ...
Got dependencies!
36 packages have newer versions incompatible with dependency constraints.

# After
Got dependencies!
36 packages have newer versions incompatible with dependency constraints.

Implementation notes

• Filter functions follow the same pattern as pytest_cmd.rs (plain text parsing, no JSON)
• Flutter test uses the compact reporter format (HH:MM +N: / HH:MM +N -M: prefix detection)
• Flutter analyze uses - separator; summary line comes from stderr
• Fixtures captured from real Flutter 3.x project output (compact test reporter)
• Command registered in src/main.rs with FlutterCommands subcommand enum
• Rewrite rule added to src/discover/rules.rs

Test plan

• rtk flutter test on passing suite -> single [PASS] All tests passed line
• rtk flutter test with failing test -> failure block only, no progress lines
• rtk flutter analyze with warnings and info -> warnings shown, info suppressed
• rtk flutter analyze clean -> [PASS] No issues found
• rtk flutter build apk -> final built line only
• rtk flutter pub get -> summary + compat warning only
• rtk rewrite "flutter test" -> prints rtk flutter test
• cargo test flutter -> 22 unit tests pass

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