feat(fragments): add command with open-changelist append + per-branch impact caps

[manuc66]

Summary

Solves the "too many fragment files per MR" friction and adds a per-branch impact cap. Stacked on #1 (gates + API-surface dogfooding): the diff shrinks to this feature alone once #1 merges.

What changed

1. changesharp add — the open changelist

• add appends a change to the most recent fragment (the open changelist), so a feature branch accumulates into a single file.
• Trunk-safe: on the default branch (main/master) add always creates a separate file → concurrent pushes stay conflict-free.
• Escape hatches: --separate (new file), --fragment <file> (specific target), --changelist <name> (deterministic named changelist).
• new remains equivalent to add --separate (backward compatible).
• The MaxImpact cap applies to add exactly as to new.

2. Per-branch impact caps (SemverPolicy.BranchMaxImpact)

{ "SemverPolicy": { "MaxImpact": "minor", "BranchMaxImpact": { "release/*": "patch" } } }

• On a release/1.2 branch, an Added (Minor) fragment is refused at add and release (exit 3) unless --allow-major; Fixed (Patch) is fine.
• Effective cap = min(global MaxImpact, matching branch entry). * suffix wildcard supported.

3. MCP parity

create_fragment gains separate / fragment / changelist params (same semantics as the CLI).

Tests

• AppendFragment (append same/new section, create, dedup, changelist/fragment/separate targets, default-branch vs feature-branch via git) and BranchMaxImpact (matching/non-matching branch, gate messages): 16 new tests → 130 total, all green.
• Baselines regenerated (cli-help.txt, mcp-tools.json, public-api.txt); the api-surface job derives minor (additions only).
• Demo samples/maximpact-gate/run-demo.sh: 11/11.

Verified manually

• Feature branch: 2 × add → 1 file, 2 sections.
• main: 2 × add → 2 files (trunk-safe).
• release/1.0 + BranchMaxImpact: release/* → patch: add --added exit 3 ("above the allowed impact (patch)"), add --fixed OK.

[manuc66]

Reopening to trigger CI after base retarget to main.

[codecov]

Codecov Report

❌ Patch coverage is 90.54545% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.86%. Comparing base (07d4d25) to head (84c7a2a).

Files with missing lines	Patch %	Lines
ChangeSharp/WorkspaceManager.cs	85.89%	9 Missing and 13 partials ⚠️
ChangeSharp/NextVersionComputer.cs	82.60%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
+ Coverage   78.33%   80.86%   +2.53%     
==========================================
  Files          12       14       +2     
  Lines        1154     1406     +252     
  Branches      194      237      +43     
==========================================
+ Hits          904     1137     +233     
- Misses        183      193      +10     
- Partials       67       76       +9     

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