inception phase 4: wire start-epic to inception

[leeovery]

Summary

Phase 4 of the inception/discovery-map initiative — first user-visible flip of the new flow.

• /start-epic Step 3 collapses to a 2-option menu: continue to inception (default) or i/import first.
• Step 4 always invokes /workflow-inception-entry epic {work_unit} (single row).
• i/import route preserved but rerouted: imports land in .workflows/{wu}/imports/ and are recorded on manifest.imports[] ({path, imported_at}). KB indexing comes in Phase 8.
• Session-loop Section A gets a third branch: when Topics Identified is empty and imports exist, the inception session reads imports and reflects them as the launchpad rather than the generic "tell me what to build" prompt.

After this PR, new epics go through inception when started.

Test plan

• bash tests/scripts/test-workflow-manifest.sh — passes (214/214 — verified locally)
• node skills/continue-epic/scripts/discovery.cjs runs cleanly on a fresh dir (verified locally)
• End-to-end smoke (no import path): /start-epic → describe → name → land directly in inception session; topics persist with right shape; bridge fires; /continue-epic doesn't error
• End-to-end smoke (import path): i/import → seed file → imports/ populated → manifest.imports[] has entry → inception opens with import content reflected
• Inbox-path smoke: positional arg → inbox content used as description → continue-or-import menu still appears → on continue, inception opens

Out of scope

Map render in continue-epic (Phase 5), KB indexing of imports + feature import behaviour change (Phase 8), removing route-first-phase.md (Phase 12).

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