Restructure agent instructions and skills

[gtsiolis]

Motivation

The agent-facing instructions had drifted from the code (nonexistent APIs in skills, an incomplete architecture map) and the root CLAUDE.md had grown past 4k words of always-loaded context, much of it deep per-feature reference.

Changes

• Root CLAUDE.md goes from 4,235 to 3,158 words (-25%): ~1,650 words of deep reference moved into nested files (loaded only when working in those packages) while ~570 words of new conventions were added
• Restructure CLAUDE.md: complete the internal/ package map and command groups, correct the test-integration and mock-generation claims (mockgen via go:generate), and add sections for commit/PR/Linear conventions, the release process, spec-driven (openspec) changes, and human-facing writing style, plus notes on the single-test/lint/JUnit workflows
• Move deep reference sections (snapshots, Azure setup/interception, volume mounts, GATEWAY_LISTEN + offline fallbacks) into nested CLAUDE.md files under internal/{snapshot,azureconfig,config,container}/, each with an AGENTS.md symlink mirroring the root convention
• Refresh skills: fix add-command (the sink.Emit(output.XxxEvent{...}) pattern instead of nonexistent output.EmitXxx helpers, initConfig(nil), Cobra group registration, requireSubcommand, isInteractiveMode, testEnvWithHome), fix the review-pr events checklist (sealedEvent() marker, no emit helper), update create-pr (current Linear MCP tool name, Linear-first ticket step), and refresh add-component (SuccessColor palette constant)
• Make test-integration depend on build (one-line Makefile change) so the binary is always rebuilt before integration tests — this is what makes the updated build docs accurate; previously a stale bin/lstk could silently run after source edits

Closes PRO-365

[gtsiolis]

Thanks for taking a look, @skyrpex! Expecting the separate agent instructions in sub-folders to make better use of context, but let's revert if this does not work as expected. Cc @anisaoshafi

[gtsiolis]

Thanks for merging, @skyrpex! 🏀

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[skyrpex]

@gtsiolis I've seen this technique being mentioned a few times as well, also heard it works well hence my approval. Let's keep an eye on it! Thank you.