Move prompts into a layered data library with trigger-based assembly

Why: prompts lived inline in Go (profile const, subagent string, squad
role one-liners, tool descriptions) - every copy change shipped code,
and task guidance like meeting-minutes quality lived permanently in the
base prompt whether or not the task was at hand.

Evidence: new prompts package tests (library integrity, default
assembly, skill triggering incl. attachment gate and command bypass,
priority supersession, override chain drift, command match, frontmatter
parse); composite make test green; runtime check on a live server
(40 modules across 7 layers, override round-trip, /minutes starts a
run instead of hitting the command bus).

Impact: 40 builtin modules as embed data (5 base + 1 env + 12 task
skills + 10 agent roles + 7 tool descriptions + 3 aux templates + 2
styles); migration v8 adds prompt_modules for user overrides with
basedOnVersion drift markers; ctxasm assembles multi-segment system
messages with per-module observability; subagent/squad/tool copy now
resolve from the library with inline fallbacks; skill commands surface
in the / menu and route to runs; default profile SystemPrompt empties
onto the library base; pins deps/agent-system-core to the contract
commit.

Refs: feature/prompt-library task card; design guide in Documents.

internal/store/migrate.go

@@ -228,6 +228,15 @@ func init() {
 				`CREATE INDEX IF NOT EXISTS idx_audit_op ON command_audit(operation_id)`,
 			}},
 			down: map[string][]string{"sqlite": {`DROP TABLE IF EXISTS command_audit`}, "postgres": {`DROP TABLE IF EXISTS command_audit`}}},
+		migration{version: 8, name: "prompt_modules",
+			// 提示词库用户覆盖（core.PromptModule）：内置在 embed 库，本表只存
+			// 用户副本/覆盖行（BasedOnVersion 供上游升级 drift 提示）。
+			up: map[string][]string{"sqlite": {
+				`CREATE TABLE IF NOT EXISTS prompt_modules (id TEXT NOT NULL, owner TEXT NOT NULL, payload TEXT NOT NULL, PRIMARY KEY(id, owner))`,
+			}, "postgres": {
+				`CREATE TABLE IF NOT EXISTS prompt_modules (id TEXT NOT NULL, owner TEXT NOT NULL, payload TEXT NOT NULL, PRIMARY KEY(id, owner))`,
+			}},
+			down: map[string][]string{"sqlite": {`DROP TABLE IF EXISTS prompt_modules`}, "postgres": {`DROP TABLE IF EXISTS prompt_modules`}}},
 	)
 }