Restructure AGENTS.md to terse wayfinding, relocate release and macOS specs

[quantizor]

AGENTS.md had grown into walls of actionable specification, which the house rules keep out of a wayfinding file (that file holds invariants, one-line map entries, and keyword hooks; specification belongs in a feature doc). This restructures it and relocates the specs. Stacked on #21; review that first. Docs only, no code.

What changed

• The codebase map collapses from six wall-of-text bullets to one terse line per structural piece, each keeping its "one home" pointer (prepareSpawn is the trust gate, liveDescendants is the descendant-union home, displayPort is the shown-port home, and so on) and every file and symbol name a reader needs to navigate. The # devctl H1 becomes a plain first line, per the agent-loaded-file formatting rule.
• The make app / make dmg / signing / notarization / Release DMG workflow / Homebrew cask bump mechanics move to a new docs/releasing.md, with a one-line hook in AGENTS. AGENTS keeps the short make build / make test / smoke-script pointers.
• The macOS "Stack notes" (swift-subprocess sessions, launchd PATH/ThrottleInterval/ExitTimeOut, SMAppService status traps, the BTM ad-hoc CDHash rebind saga, the DMG/Applications shared-bundle-id rules) move verbatim to a new docs/macos-lifecycle.md, with a one-line hook.

What did not change

Every hard rule and every engineering rule stays. Those are the invariants the file exists to hold, so they were tightened in wording where the mechanism already lives in code comments, but none was dropped, weakened, or stripped of its rationale or version pin. The coverage rule keeps its exact measurement command, as the house rules require naming the command that prints the live figure.

Verification

• AGENTS.md dropped from roughly 29.5 KB to 19 KB, almost all of it the two relocated sections and the map compression.
• Every file and symbol named in the rewritten map was grepped and resolves; the relocated macOS notes preserve every specific (BackgroundItems-v16.btm, the sfltool no-sudo note, the xpcproxy invalidation, diagnose-by-UUID). No em-dashes; no H1, bold, tables, or horizontal rules in AGENTS.md.

Follow-up worth a backlog entry

The map names about fifty files and symbols and nothing checks they still exist, so a rename rots it silently. A small CI grep asserting each Sources/... path and each backticked symbol in AGENTS.md resolves would make that fail loudly. Noting it here since the maintainer-local BACKLOG.md is not in a clone.

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