fix(init): repair '(text)' config corruption and adjacent multi-agent push races (#739)

[dollspace-gay]

Summary

Five related fixes that together make a freshly-crosslink init'd repo actually round-trip between agents.

• Stops the (text) placeholder corruption — WalkthroughCore::build_config no longer writes the UI placeholder "(text)" into hook-config.json for every ConfigType::String key. Pre-fix, every new repo got tracker_remote = "(text)" (and matching corruption for both sentinel.*.model keys), causing every push to fail as RemoteMisconfigured("(text)"). populate_tracker_remote and read_tracker_remote now auto-repair / gracefully degrade on the corrupt value.
• Closes the multi-agent bootstrap race — sync_cmd now calls a new SyncManager::push_hub_if_ahead() at the end. Previously, pushes happened lazily only via SharedWriter::commit_*, so agent B's init_cache could run between A's bootstrap sync and A's first write, see an empty remote, create its own orphan branch, and then never rebase onto A's eventual push.
• Hardens the PushFailure::NonFastForward classifier — added "! [remote rejected]", "cannot lock ref", "incorrect old value provided", and "failed to push some refs" to the substring set. Pre-fix, concurrent lock claims hit a path where both agents got PushFailure::Other, both call sites returned LocalOnly, and both printed "Claimed lock on issue Support hook-config.local.json for machine-local configuration #1".
• Tolerates cpitd 0.3.0's schema rename — total_pairs → total_groups via #[serde(default, alias = "...")]. Empty-output cpitd scan no longer fails to parse. Non-empty schema migration is a separate concern.

Test plan

• 8 new unit tests added (1 in config_registry, 3 in init, 1 in sync::tests for the read fallback, 2 in sync::tests for the classifier patterns, plus the test counts come from build_config + populate_tracker_remote + classifier expansions)
• 4 previously-failing smoke tests now pass:
  • smoke::cli_tooling::test_cpitd_scan_dry_run
  • smoke::concurrency::test_parallel_lock_claim_one_winner
  • smoke::coordination::test_two_agents_create_issues
  • smoke::coordination::test_adversarial_same_issue_write_conflict_convergence
• Full suite green: 1,883 lib + 2,859 bin + 194 integ + 159 smoke pass, 0 failures
• cargo fmt --check clean
• cargo clippy --all-targets -- -D warnings introduces zero new errors (21 pre-existing errors on develop, 21 on this branch)

Recovery for existing affected repos

Repos initialized before this fix carry the corrupt config sticky — init --force from this build will auto-repair them. For pre-build repos, run:

crosslink config set tracker_remote origin
crosslink config set sentinel.default_agent.model <model>
crosslink config set sentinel.escalation.model <model>

Out of scope (follow-up)

• Full cpitd 0.3.0 schema migration to actually parse non-empty clone groups (the alias only unblocks the empty path).
• Backfilling unit tests for the new push_hub_if_ahead happy path / offline path / divergence path. The smoke tests exercise the behavior end-to-end.

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