docs: point the custom-* skills row at its activation on-ramp

[KbWen]

Summary

Closes the documentation half of #292 — the split the issue's own triage analysis recommended shipping standalone as a tiny-fix.

docs/INSTALL.md's "Customizing without conflicts" table had a one-cell asymmetry between two sibling extension seams:

• "Connect an external knowledge base" points at knowledge_sources: in .agentcortex/context/private/downstream-capabilities.yaml.
• "Add your own skills" told the adopter where to put a custom-* skill and that it survives upgrades — but not that survival is not activation.

Verified against the code, an undeclared custom-* skill is genuinely second-class:

• .agent/workflows/bootstrap.md:405 — a skill id resolves only via the §3.6 rule table, the compact index, or a custom-* id declared in downstream-capabilities.yaml §skills; anything else is "unknown → ignore". So an undeclared custom-* id is never auto-recommended and cannot be pinned from user-preferences.yaml.
• docs/adr/ADR-007-...:33-38 states the same outright ("Custom skills are second-class … cannot auto-activate").

The row now says so and points at the existing opt-in on-ramp, mirroring the shape of its sibling row (pointer + ADR link + the on-match cap).

Adopter delta: an adopter reading INSTALL now learns from the same table that a custom-* skill needs a one-line declaration to become activatable. Engine behaviour is unchanged — no new detector, no auto-discovery (still rejected by ADR-007), no always-on cost.

This deliberately does not build the runtime bootstrap detector the issue proposes; that half stays demand-gated per the issue's own report-trigger framing, so #292 is referenced, not closed.

Evidence

Validator — bash .agentcortex/bin/validate.sh:

[PASS] token lifecycle drift: within slack
Summary: pass=99 warn=4 fail=0 skip=3
Agentic OS integrity check passed
[exited with code 0]

Pinned test module (docs/INSTALL.md carries substring assertions in this file, so it was re-run rather than assumed doc-safe):

$ python -m pytest tests/ci/test_deploy_tiering.py -q
38 passed, 1 skipped in 343.50s (0:05:43)

Table integrity re-checked after the edit (14 table rows before and after; single-line diff, no merged rows). Link target docs/adr/ADR-007-downstream-capability-declaration-seam.md confirmed to exist.

Scope: 1 file, 1 line changed, doc-only.

Refs #292

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