feat(mentat): skill distribution to agent-specific paths

[bketelsen]

Summary

After mentat sync generates per-domain SKILL.md files, the new distributor package copies each skill to every agent toolchain's expected location in a single pass.

What's added

mentat/internal/distributor/distributor.go

Five distribution targets (all enabled by default via DefaultConfig()):

Target	Output path	Transformation
pi	.agents/skills/{domain}/SKILL.md	as-is
claude	.claude/commands/{domain}.md	frontmatter stripped
codex	.codex/skills/{domain}.md	frontmatter stripped
cursor	.cursor/rules/{domain}.mdc	description-only frontmatter, .mdc extension
agents-md	AGENTS.md (repo root)	all domains, sorted ## {domain} sections, no frontmatter

Public API:

• Distribute(repoPath, domain, content string, cfg Config, log *slog.Logger) error — single domain, all targets except agents-md
• DistributeAll(repoPath string, skills []SkillContent, cfg Config, log *slog.Logger) error — full set, then regenerates AGENTS.md
• Respects clix.DryRun; logs intent without writing

mentat/internal/distributor/distributor_test.go

23 table-driven tests covering: all five target paths, frontmatter stripping/rewriting, Cursor description extraction, AGENTS.md ## sections + no-frontmatter + alpha sort, disabled targets, dry-run (no writes), and content with no frontmatter.

mentat/cmd/mentat/commands.go

• Wires distributor.DistributeAll into syncCmd after generator.GenerateAll returns
• Reads back written SKILL.md files and passes []distributor.SkillContent to the distributor
• Adds --no-distribute flag to skip distribution (useful for quick stale-file reruns)

docs/exec-plans/active/mentat-skill-distribution.md

Exec plan per AGENTS.md convention (3+ files changed).

Verification

just build-mentat   # exit 0
cd mentat && go test ./...   # all packages pass
cd mentat && go vet ./...    # clean