docs(walkthrough): add §5.2 comparing none / warn / exclusive policies

[albertgwo]

Purpose

Give operators an explicit decision table for --policy (none, warn, exclusive) so downstream projects do not invent their own avoidance guidance. The assign update PR (#24) surfaced a recurring failure mode: a downstream AGENTS.md steered workers away from exclusive on race grounds, but the race in question was closed by v0.1.2's BEGIN IMMEDIATE transaction. Without a kernel-side comparison, each consumer reverse-engineers policy semantics from SPEC §15 and §28 separately, and stale guidance persists across consumer repos.

Summary of changes

• New docs/walkthrough.md §5.2 "Choosing a conflict policy" with a per-policy table covering overlap detection, outcome, lock sentinel, and an operator-facing "when to use" column.
• Explicit note that all three policies are race-safe under concurrent writers since v0.1.2, with a code citation (internal/domain/domain.go ResolveAndInsertIntent) and a pointer to the tests/integration/concurrent_test.go suite that keeps the guarantee honest under -race.
• Explicit call-out for downstream projects whose AGENTS.md still avoids exclusive on race grounds: verify the installed agent-ledger --version is at least v0.1.2 and update local guidance.
• Note that the <ledger-dir>/locks/<path-hash>.lock sentinel is advisory (DB row is authoritative per SPEC §28) and is housekeeping for the verifier, not a public API.

Changes to review

• docs/walkthrough.md §5.2: confirm the table's "when to use" column matches how you actually want operators to choose between the three policies.
• Race-safety claim: the section cites v0.1.2 (2026-04-28) as the cut-over. Confirm that v0.1.2 is the canonical version to name (vs. v0.1.0 or a later patch).
• Tone: the section is direct about stale operator guidance. Confirm the framing is appropriate for the project's voice; soften if it reads too prescriptive for a walkthrough document.