fix(strategy): require fetch URLs for checkpoint sync remote election

[MuskanPaliwal]

Closes #2360

Trail: https://entire.io/gh/MuskanPaliwal/cli/trails/4

A Git remote can be configured with only a pushurl and no fetch URL. Such a remote can receive pushes, but Entire cannot read checkpoint history or reconcile checkpoint state from it.

ResolveCheckpointSyncRemote previously treated this configuration inconsistently:

• Explicit and previously captured remote selections were validated with git remote get-url.
• Git reports success for a pushurl-only remote—even returning the remote name when no fetch URL exists.
• Automatic election used remote.*.url entries and therefore correctly excluded the same remote.

This allowed an explicit checkpoint_push_remote to select a destination that automatic election considered unusable. entire status then presented that remote as the active checkpoint destination even though checkpoint reads and reconciliation could not work against it.

Use one eligibility rule throughout election

Remote election now builds the set of remotes with configured fetch URLs once and uses that set for every election tier.

The behavior differs deliberately based on who owns the selection:

Election source	Previous behavior	New behavior
Explicit checkpoint_push_remote	Elected a pushurl-only remote	Fails closed and explains that the remote has no fetch URL
Captured automatic selection	Re-elected a pushurl-only remote	Discards the stale selection and falls through to normal defaults
Automatic default election	Already excluded pushurl-only remotes	Unchanged
Gated-sync hint	Could recommend a pushurl-only remote	Stays silent because the suggested destination would be unusable

An explicit setting fails closed because it represents a user decision that should be corrected rather than silently replaced. Captured state remains fail-soft because it was produced automatically and may become stale when remotes are renamed or reconfigured.

Status and diagnostics

When the explicit remote has no fetch URL, entire status now:

• Reports that checkpoints are not syncing.
• Names the invalid remote.
• Explains that it lacks a configured fetch URL.
• Does not also print a misleading Checkpoints sync to: destination.

The gated-sync hint uses the same fetchability rule, ensuring it never recommends a destination that checkpoint sync cannot read from.

Regression coverage

The new tests construct real isolated Git repositories with:

[remote "pushonly"]
    pushurl = https://example.com/pushonly.git

and deliberately omit remote.pushonly.url.

They verify that:

• An explicit pushurl-only remote fails closed.
• The error names the remote and its missing fetch URL.
• A captured pushurl-only remote falls back to origin.
• Status reports the failure without advertising a sync destination.
• The gated-sync hint remains silent for the unusable remote.

Verification

The following checks passed:

• go test ./cmd/entire/cli/strategy -run '^TestHintGatedCheckpointSync$'
• go test ./cmd/entire/cli/strategy
• mise run check
  • formatting
  • lint
  • race-enabled unit and integration tests
  • Vogon E2E canary
  • Roger Roger E2E canary
• mise run lint