fix(config): silently infer tracker_remote from git remotes (GH#611)

[dollspace-gay]

Summary

• Closes GH#611 — WARN no tracker_remote configured in .../hook-config.json, defaulting to "origin" was firing on every process invocation in any project that hadn't explicitly set tracker_remote.
• Replaces the WARN-and-default with silent inference from git remote. Common case (single origin remote, no config) now resolves silently with zero output.
• Keeps explicit hook-config.json values winning unconditionally, and keeps the "(text)" corruption sentinel (#739) — but now falls through to inference instead of blindly returning "origin".

Why this matters beyond the noise

Per #611 the WARN piles up across kickoff agents, hooks, and interactive use, and apparently triggers a downstream auto-mutation in some environments that has been observed to corrupt other keys in hook-config.json. Removing the WARN removes the trigger without changing the semantics for already-correct configs.

Resolution order (new behavior)

1. Explicit tracker_remote in hook-config.json → use verbatim (unchanged).
2. "(text)" placeholder (GH#739) → one-shot WARN, fall through to inference (was: return "origin").
3. Inference from git remote:
   • exactly one remote → use it
   • multiple including "origin" → prefer "origin"
   • multiple without "origin" → first alphabetical (deterministic across machines)
   • zero remotes → one-shot WARN + "origin" (the one case where a real diagnostic is warranted, since sync won't work)

Changes

• sync/mod.rs — new infer_tracker_remote + list_git_remotes helpers; read_tracker_remote reroutes through them. Module gains a std::process::Command import (was pure config read, now invokes git remote on the missing-field path). Doc-block on read_tracker_remote rewritten to describe the new resolution order.
• sync/tests.rs — 5 new tests covering single-origin (silent), single non-origin, multi-with-origin, multi-without-origin (alphabetical), and explicit-config-wins. The 4 pre-existing tests are unchanged and still pass.

Test plan

• 5 new unit tests added covering all inference branches.
• All 4 pre-existing read_tracker_remote tests still pass.
• cargo test --lib — 1904 passed, 0 failed.
• cargo clippy -- -D warnings -W clippy::unwrap_used -W clippy::expect_used clean.
• cargo fmt --check clean.
• Manual smoke test: ./target/debug/crosslink session status against this repo no longer emits the WARN (this repo has no tracker_remote set but has an origin remote).

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