fix(import): clarify conflict diff behavior (#359)

[luongnv89]

Closes #359
Closes #360

Summary

Make import conflict behavior explicit and testable: asm import --force --diff now fails before reading or modifying a manifest, while the interactive and non-interactive conflict display paths have focused regression coverage.

Approach

Selected the minimum-risk validation approach. --force remains an unconditional overwrite mode with no conflict construction, and --diff remains a conflict-inspection mode; combining them is rejected as invalid CLI usage rather than changing importer contracts or silently ignoring --diff. The prompt and non-interactive renderer were extracted with injectable I/O seams so their existing behavior can be tested without a real TTY.

Decision Record

• Root cause: Force imports bypass conflict construction, so the later non-interactive diff loop receives no conflict records and silently renders nothing; the prompt and rendering glue were nested inside cmdImport, leaving no practical unit-test seam.
• Options considered: Option 1 — document that --diff is ignored with --force; Option 2 — reject the incompatible flags before import work; Option 3 — construct reporting conflicts during forced overwrites.
• Options rejected: Option 1 preserves a surprising silent no-op; Option 3 changes importer behavior and risks rendering only after destructive writes.
• Selected option: Option 2 — reject the incompatible flags and extract focused conflict UI seams.
• Residual risk: A real pseudoterminal interaction is not exercised; prompt parsing, rendering, and callbacks are covered through the same functions used by cmdImport.

Analyzed at: feat/359-import-force-diff @ 1cdee2b (2026-07-11)

Changes

File	Change
src/cli.ts	Reject --force --diff before manifest I/O, document the incompatibility, and expose testable prompt/diff helpers.
src/cli.test.ts	Cover k/u/s/d prompt choices, newer/older labels, interactive diff display, real non-interactive conflict diff output, and early incompatible-flag rejection.

Test Results

• Unit/integration tests: 1,895 passed (npm test)
• Focused CLI tests: 364 passed
• Typecheck: passed
• Build: passed
• E2e node tests: passed in pre-push hook
• Formatting: passed
• QA cycles: 2

Acceptance Criteria Verification

Criterion	Status	Evidence
--force --diff no longer silently renders nothing	pass	CLI integration: import > import rejects --force with --diff before reading the manifest
Rejection occurs before import side effects	pass	The regression uses a nonexistent manifest and asserts the incompatibility error precedes file reading.
k/u/s prompt choices map to keep-local/use-imported/skip	pass	import conflict prompt > maps ... parameterized tests in src/cli.test.ts
d prints a diff and re-prompts	pass	import conflict prompt > shows a diff for d, then prompts again for a resolution
Newer/older sides are marked correctly	pass	import conflict prompt > marks the ... side newer and the other side older
Non-interactive --diff prints actual conflict output without overwriting	pass	CLI integration: import > import --diff prints a collected conflict in non-interactive mode