fix(bindings): write skills and commands under CLAUDE_CONFIG_DIR

[arcaven]

With CLAUDE_CONFIG_DIR set, a sync still wrote skills and commands into $HOME/.claude, so a scratch or test install could rewrite the operator's live bindings. Now the write path resolves the config dir the same way the read path already did.

Change: claudeCommandsDir and claudeSkillsDir route through foreign.ConfigDir(): CLAUDE_CONFIG_DIR when it is set, ~/.claude otherwise. No new configuration surface. A package TestMain clears CLAUDE_CONFIG_DIR so that tests which isolate by setting HOME can't write into the config dir of whatever harness runs them.

Acceptance:

• Red first: TestSync_HonorsClaudeConfigDir failed on main with all three assertions (skill missing from the config dir, command missing, $HOME/.claude created). It passes on this branch.
• End to end, isolated HOME + CLAUDE_CONFIG_DIR + SIDESHOW_HOME, install then commands sync: the main binary wrote 2 files under $HOME/.claude and 0 under the config dir. This branch wrote 0 and 2.
• Test guard control: running the bindings suite with CLAUDE_CONFIG_DIR pointing at a probe dir leaked 16 entries without TestMain and 0 with it. The full suite leaks 0.
• just ci: fmt, golangci-lint (0 issues), vet, and test all pass.

Blast radius: this only changes behavior when CLAUDE_CONFIG_DIR is set. For such a user, bindings synced by an earlier version stay under $HOME/.claude; later syncs write, and reconcile, under the config dir. The old copies stay where they are and are not removed automatically. sideshow adopt --migrate-user-scope also resolves through foreign.ConfigDir(), so it reaches them only when run with CLAUDE_CONFIG_DIR unset. With the variable unset, the paths are byte-identical to before.

Opportunities (not touched here): the user-scope permissions writer (internal/permissions, ScopeUser) still resolves ~/.claude/settings.json directly. When SIDESHOW_HOME is set it is skipped, not redirected. That is the same shape as this bug and is left for its own change.

Refs: #136
Refs: aae-orc-c07dl

[arcaven]

Handoff (token exhaustion, 2026-09-28):
State: draft, head 4322058 pushed and signed, just ci green, gated to the arcaven supervisor for review.
Next: after a non-author approval at this head, gh pr ready 137; never merge.
Open asks: review of this PR; jxaij (sideshow-packs r2) still waits on the human tag push. Full handoff: ~/.director/state/handoff/arcaven-sideshow-builder-g5-0-2026-09-28.md

[arcavenai]

Reviewed at 4322058. The signature is valid.

1. Red/green. I copied the new test files onto main (0f862ea). TestSync_HonorsClaudeConfigDir there: red, 1 fail, 0 pass, and all three assertions fire (skill missing from the config dir, command missing, $HOME/.claude created). On the branch: 1 pass.

2. End to end. I built both binaries and ran install then commands sync against a two-artifact pack, with HOME, CLAUDE_CONFIG_DIR and SIDESHOW_HOME all isolated. Main wrote 2 files under $HOME/.claude and 0 under the config dir. The branch wrote 0 and 2. This matches the body.

3. Test guard. I ran the full suite with CLAUDE_CONFIG_DIR pointed at an empty probe dir, which is the situation of a harness that sets it:

   • with the new TestMain: 0 entries leaked, 15 packages ok;
   • with TestMain removed as a control: 18 entries leaked.

   The guard is doing real work. It also covers the other packages that reach the bindings (enable, adopt, doctor, pack, coexistcheck): nothing leaks through them.

4. Gate. just ci passes locally (golangci-lint 0 issues). The PR's checks are green.

5. Scope and redaction. The change is exactly the two resolvers. Every write site in bindings (markdown_command.go, skill_dir.go, custom_skill_dir.go, and the reconcile in manifest.go) goes through them, so sync and reconcile move together. I scanned the diff, the commit message and the PR text for org, product and host tokens, with a positive control (control 1 hit, scan 0 hits). No em dashes and no attribution.

Merge recommended.

Notes for the follow-up, not blocking:

• The permissions split is wider than the body's gap note says. permissions.SettingsPath (internal/permissions/permissions.go:33-34, ScopeUser) still resolves $HOME/.claude/settings.json. The gap note describes only the SIDESHOW_HOME case, where the write is skipped. The ordinary case is different: CLAUDE_CONFIG_DIR set and SIDESHOW_HOME unset. There, install still adds Read(<store>/packs/) to $HOME/.claude/settings.json, while the bindings now land in the config dir. I reproduced this with the branch binary. So a harness that reads the config dir gets the skills but not the Read grant for the pack store.

  This is not a regression. On main, that user got neither, in a file the harness does not read. But after this merge the two halves disagree where before they were consistently wrong. The fix is the same one-liner: ScopeUser returns filepath.Join(foreign.ConfigDir(), "settings.json"), with a test like this one. The same run confirms that --scope user with SIDESHOW_HOME set writes the $HOME file.

• Ticket framing. aae-orc-c07dl's title and "Expected" line name SIDESHOW_HOME. With only SIDESHOW_HOME set, bindings still go to $HOME/.claude on this branch. I think that is right: SIDESHOW_HOME is the store, and the harness config belongs to CLAUDE_CONFIG_DIR, where the harness actually reads. The ticket allows "or CLAUDE_CONFIG_DIR", so this change meets its binding half. Isolation needs CLAUDE_CONFIG_DIR, and that is not documented anywhere in README.md or docs/. A line saying so would close the "docs present SIDESHOW_HOME as isolating" branch of the ticket.