feat(agents): enforce size budget + extract duplicated boilerplate (#2361) (#2362)

Adds tiered agent-size-budget test to prevent unbounded growth in agent
definitions, which are loaded verbatim into context on every subagent
dispatch. Extracts two duplicated blocks (mandatory-initial-read,
project-skills-discovery) to shared references under
get-shit-done/references/ and migrates the 5 top agents (planner,
executor, debugger, verifier, phase-researcher) to @file includes.

Also fixes two broken relative @planner-source-audit.md references in
gsd-planner.md that silently disabled the planner's source audit
discipline.

Closes #2361

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

get-shit-done/references/mandatory-initial-read.md

@@ -0,0 +1,2 @@
+**CRITICAL: Mandatory Initial Read**
+If the prompt contains a `<required_reading>` block, you MUST use the `Read` tool to load every file listed there before performing any other actions. This is your primary context.

get-shit-done/references/project-skills-discovery.md

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+# Project Skills Discovery
+
+Before execution, check for project-defined skills and apply their rules.
+
+**Discovery steps (shared across all GSD agents):**
+1. Check `.claude/skills/` or `.agents/skills/` directory — if neither exists, skip.
+2. List available skills (subdirectories).
+3. Read `SKILL.md` for each skill (lightweight index, typically ~130 lines).
+4. Load specific `rules/*.md` files only as needed during the current task.
+5. Do NOT load full `AGENTS.md` files — they are large (100KB+) and cost significant context.
+
+**Application** — how to apply the loaded rules depends on the calling agent:
+- Planners account for project skill patterns and conventions in the plan.
+- Executors follow skill rules relevant to the task being implemented.
+- Researchers ensure research output accounts for project skill patterns.
+- Verifiers apply skill rules when scanning for anti-patterns and verifying quality.
+- Debuggers follow skill rules relevant to the bug being investigated and the fix being applied.
+
+The caller's agent file should specify which application applies.