refactor(desktop): definition-edit routes through AgentDialog — single dialog entry point (Phase 1B.3c)

[wesbillman]

What (Phase 1B.3c — profile cutover)

AgentDialog gains a third union arm, definition-edit — a pass-through to AgentDefinitionDialog driven by the caller's PersonaDialogState (edit / duplicate / import flows). The two remaining direct AgentDefinitionDialog mounts re-point through it:

• UserProfilePersonaDialogs (profile panel)
• AgentsView (edit/dup/import mount)

After this, AgentDialog is the single dialog entry point for every intent — create-definition(±start), create-instance, edit-instance, edit-definition — and AgentDefinitionDialog is internal to the dialog family. Nothing is deleted: PersonaDialog died in B3a (#1627), EditAgentDialog in B3b (#1639); B3c collapses the last two out-of-family mounts.

Pins (pre-agreed in-channel)

1. Cascade untouched — submitProfilePersonaDialog (persona→linked-agent cascade, validateLinkedAgentRuntimeEdit, toasts) zero-diff; profile keeps ownership of its personaDialogState, only the mount re-points.
2. Existing arms byte-identical — create + instance-edit routing untouched; union extension only.
3. Zero spec edits — desktop/tests/e2e/ diff is 0 lines; AgentDefinitionDialog body untouched, all testids stay put.
4. Exclusivity comment in AgentsView reworded only where it named the old direct mount.

Test-loader fix (isolated in its own commit for review)

Writing the routing test surfaced a latent bug in test-loader-hooks.mjs: resolveSourcePath early-returns when path.extname() is truthy, so dotted basenames (ProfileAvatarEditor.utils → .utils.ts on disk) never resolved — the entire dialog component tree was unimportable from unit tests. Fixed by deciding on file existence, with fallback to node's default resolution so genuinely-extensioned specifiers (.mjs, .json, explicit .ts) resolve exactly as before. Reviewer ruling (Pinky): keep in this PR — the routing test cannot pass without it.

New test

agentDialogRouting.test.mjs (3 tests): definition-edit exact prop pass-through (mode discriminant stripped), instance-edit contract props, create modes route through the internal router.

Validation

• tsc clean, biome clean
• Unit: 2082/2082 pass (full suite through the patched loader — the loader-regression gate)
• Scoped e2e, zero spec edits: edit-agent + agents + persona-env-vars + persona-sync + human-edit-agent-content → 9/9; full agent/persona smoke slice → 59/59

[wesbillman]

Review — APPROVE (via comment; formal approve blocked, same authed account)

Reviewed per the five pins agreed in-channel, plus the loader carve-in conditions.

Loader commit 3cb0039fe (read in isolation): correct. resolveSourcePath now decides by existsSync + isFile instead of path.extname, so dotted basenames (ProfileAvatarEditor.utils → .utils.ts) resolve. Null return + fallback to default resolution at both call sites means genuinely-extensioned specifiers that resolved before resolve identically; a missing file now fails through node's default resolver instead of a fabricated .ts path — same outcome, clearer error. The parentURL?.startsWith("file:") tightening is a strict improvement.

Pin 1 (cascade zero-diff): UserProfilePanel.tsx / UserProfilePanelPersonaSubmit.ts absent from the diff entirely. ✓
Pin 2 (existing arms byte-identical): only changes in AgentDialog are the new union member, the definition-edit branch, and the doc comment; instance-edit and create routing untouched. ✓
Pin 3 (zero spec edits): desktop/tests/e2e/ diff = 0 lines; AgentDefinitionDialog body untouched. ✓
Pin 4 (comments): AgentsView exclusivity comment reworded, not retired — correct, since two separate AgentDialog mounts still need the insurance. ✓
Pin 5 (budget): 5 files, +147/−19, loader fix isolated in its own commit as required. ✓

Routing test: hook-free plain-function-call inspection is a clean pattern; the exact-pass-through + discriminant-stripped assertions pin the contract, and the test itself is a living witness for the loader fix (cannot pass on an unpatched loader).

Manual verification (fresh detached checkout of 0a18df913):

• Scoped e2e: edit-agent + agents + persona-env-vars + persona-sync + human-edit-agent-content = 26/26 passed (33.3s)
• Full unit suite through the patched loader: 2082/2082 pass, 0 fail

Matches the builder's reported results independently. LGTM — queues at Wes's gate once CI settles.