add change-coupling report (--coupling / --coupling-for)

[boyter]

Adds a change-coupling report derived from git history which files tend to change together in the same commits.

[Copilot]

Pull request overview

Adds a new git-history “change coupling” report to show which files tend to change together (repo-wide pair overview via --coupling, and per-file “blast radius” via --coupling-for), with MCP and CLI flag wiring. The PR also adjusts several existing history report renderers to better fit the 79/109-column tabular layouts and refine what gets shown.

Changes:

• Add change-coupling analysis + renderers (tabular/csv/json) and a dedicated test suite.
• Wire --coupling / --coupling-for into CLI processing and expose a new MCP coupling tool.
• Tweak history report tabular outputs (hotspots, authors, author timeline, languages timeline) for wide layouts and/or row selection.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
processor/processor.go	Adds coupling globals + dispatch, enforces report exclusivity, hooks coupling into history-flag validation flow.
processor/history_languages.go	Changes languages timeline tabular rendering to show all rows (removes prior top-N cap).
processor/history_hotspots.go	Adjusts wide hotspots formatting widths and changes tabular filtering to skip zero-score rows.
processor/history_coupling.go	New coupling report implementation: co-change counting, rename folding, target resolution, and renderers.
processor/history_coupling_test.go	New tests covering target-path normalization, counting, rename folding, and partner ranking behavior.
processor/history_authors.go	Adjusts wide author table formatting to better fill the wide-column rule and show longer names.
processor/history_author_timeline.go	Improves tabular author timeline: wide name column and “others (N)” aggregation beyond top-N.
mcp.go	Adds MCP coupling tool and handler (blast-radius JSON) with depth/limit controls.
config.go	Registers --coupling and --coupling-for flags.

Comments suppressed due to low confidence (1)

processor/processor.go:929

• validateHistoryFlags currently returns early unless one of Hotspots, ByAuthor, or Timeline is set (see processor/history_validation.go:17). With --coupling enabled, this call won’t validate --depth or emit the standard warnings (extra positional paths, ignored flags), even though coupling is also a history report. The validation gate should include Coupling (and likely CouplingFor) so coupling runs get the same safety checks/warnings.

	if Hotspots || Coupling || ByAuthor || Timeline {
		if err := validateHistoryFlags(os.Stderr); err != nil {
			fmt.Println(err)
			os.Exit(1)
		}

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[boyter]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Addressed in b53a09f.