docs: add buildstream skill for factory-wide BST patterns

[castrojo]

What

Adds docs/skills/buildstream/ — a factory-level skill capturing the BuildStream 2 patterns that are true for every factory BST repo (today: dakota and fsdk-containers), harvested from both repos' docs/skills/.

• SKILL.md — scope rule, the per-repo divergence table, 7 core patterns, red flags, verification
• references/cache-keys-and-junctions.md — strong/weak key model, invalidation cones, junction drift-control policy
• references/element-authoring.md — element/source kinds, dependency types, variables, conditionals, sandbox constraints, overlap rules
• Registered in docs/SKILL.md router; index.json/index.md regenerated; /apache/buildstream added to the known Context7 IDs in context7.md

Junction patch queues: drift control, not "never patch"

A junction's patch_queue is part of its source hash, so the queue determines which upstream artifact cache your keys align with:

• Patches diverging from the parent project's own queue are cache-destroying → prohibited.
• Patches replicating the parent's queue byte-for-byte at the pinned ref are cache-aligning → mandatory when the parent carries one.
• The enforceable rule is drift control against the parent's queue (dakota: just patch-drift-check in CI); the per-repo parameter is which upstream queue to match (currently gnome-build-meta at the pinned ref in both repos).

Note: dakota's own docs/skills/patch-junctions.md states an absolute "NO LOCAL JUNCTION PATCH QUEUES" rule that is contradicted by dakota's current state (patches/freedesktop-sdk/ carries 0001–0007) and by its own bst-overrides.md (byte-identity requirement). That inconsistency is reported for correction at the source in dakota — this skill deliberately does NOT repeat the absolute rule.

The general-vs-repo-specific line

Promoted only what holds in both repos. Explicitly documented as per-repo decisions, not factory rules:

Decision	dakota	fsdk-containers
Composition	bootable OS, platform content in scope	components/* only, platform.bst banned
Micro-arch	opt-in x86_64_v3	banned (broad-compat baseline)
Output	bootc OS (systemd/shell/dconf/ldconfig)	distroless, no shell, SLIM + just verify

Not promoted: dakota's packaging templates (packaging-*.md), systemd/preset/dconf/bootc-ldconfig rules, GNOME/GL specifics; fsdk-containers' SLIM recipe, no-shell rule, verify gates, FSDK-tag versioning.

Evidence

• BST behavior verified via Context7 /apache/buildstream (arch_cachekeys.md, format_public.md, format_project.md, handling-files/composition.md, downloadablefilesource.py); cited inline.
• Repo claims verified against source: project.conf options in both repos, fsdk integration/ldconfig.bst usage, dakota elements/freedesktop-sdk.bst v3 conditional, both repos' junction patch queues.
• pre-commit run --all-files green (front-matter, index staleness, doc links).

Docs-only change (docs/**); opened as draft for review per factory PR policy.