/api/slots/{name}/config returns 404 (not 400) for unknown slot

[thinmintdev]

What to build

GET /api/slots/doesntexist returns 404 with code slot.not_found, but GET /api/slots/doesntexist/config returns 400 with code slot.config_error: slot config /etc/hal0/slots/doesntexist.toml not found and no in-memory state. A UI distinguishing "slot doesn't exist" from "slot exists but config is bad" gets ambiguous signals.

Map "no config file AND no in-memory state" to SlotNotFound (404) upstream in the slot manager's get_config() method.

Acceptance criteria

• GET /api/slots/doesntexist/config returns 404 with code slot.not_found
• An existing slot with a syntactically invalid TOML config still returns 400 with code slot.config_error
• Unit test covers both branches

Blocked by

None - can start immediately

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Sourced from tests/harness/FINDINGS.md §19 (low).