feat: rename pass_mgr → secret-mgr, add ego-mgr binary and skill

M1: Rename pass_mgr to secret-mgr
- Rename directory, binary, and Go module
- Update install.mjs to build/install secret-mgr
- Update pcexec.ts to support secret-mgr patterns (with legacy pass_mgr compat)
- Update plugin config schema (passMgrPath → secretMgrPath)
- Create new skills/secret-mgr/SKILL.md
- install.mjs now initializes ego.json on install

M2: Implement ego-mgr binary (Go)
- Agent Scope and Public Scope column management
- Commands: add column/public-column, delete, set, get, show, list columns
- pcexec environment validation (AGENT_VERIFY, AGENT_ID, AGENT_WORKSPACE)
- File locking for concurrent write safety
- Proper exit codes per spec (0-6)
- Agent auto-registration on read/write
- Global column name uniqueness enforcement

M3: ego-mgr Skill
- Create skills/ego-mgr/SKILL.md with usage guide and examples

Ref: REQUIREMENTS_EGO_MGR.md

ego-mgr/go.mod

@@ -0,0 +1,10 @@
+module ego-mgr
+
+go 1.24.0
+
+require github.com/spf13/cobra v1.8.0
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.5 // indirect
+)