docs: lead AGENTS.md first principle with integrated-over-time framing

Reframes the first-principle section to lead with Winters' "engineering
is programming integrated over time" as the lens, keeping "minimize
ongoing liability" as the operative directive and folding in "complexity
should be earned." Adds a new Tiebreakers subsection with two rules
that the prior section lacked clean appeals for:

- correctness > simplicity > performance (lexicographic)
- reversibility shapes evidence demand (reversible → prod metrics over
  napkin math over RFCs; irreversible → RFC up-front)

Adds a Hyrum's-Law deny-list entry in both AGENTS.md and
docs/invariants.md §IX: shipping observable behavior is shipping a
contract, even when undocumented.

Net always-on context cost: ~7 lines. No renumbering of §I–VIII
invariants; Hyrum's Law lands in the deny-list to avoid breaking
back-references.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/invariants.md

@@ -220,6 +220,7 @@ If a proposal fits one of these, the burden is on the proposer to justify why th
 - **Hand-rolling something the substrate already does.** Check the spec first (§I.1).
 - **Mutating in place** state that should be immutable (Lance fragments, index segments). New segments instead.
 - **Silent failures.** OOM, timeout, partial result — all surfaced and bounded (§V.20).
+- **Shipping observable behavior as if it weren't part of the contract.** Output ordering, error-message text, timestamp precision, default-flag values, latency profile, query-result column order — every observable behavior gets depended on once shipped (Hyrum's Law). Don't expose what you don't want to commit to; treat changes to undocumented-but-observable behavior as breaking changes.
 - **Strict-serial coordination expressed as locks held across queries.** Branches are the agent-native primitive for that (§VI.35).
 - **Auto-resolving merge conflicts when the per-type policy is silent or absent.** Pause and surface the conflict; never silently pick a side (§VI.37).