install: fix release download routing and add Codex skill

[justrach]

Summary

• fix hosted versioned download routing by redirecting exact asset URLs to GitHub releases
• update repo links and source-build docs to use justrach/codedb
• add an in-repo Codex skill for installing or building codedb and registering it as an MCP server

Verification

• deployed updated install worker to codedb.codegraff.com
• verified the versioned asset URL returns a 302 to the exact GitHub release asset
• verified curl install flow completes successfully
• verified the installed binary starts and can run CLI commands
• tested the skill helper script in an isolated HOME

[github-actions]

Benchmark Regression Report

Threshold: 10.00%

Tool	Base (ns)	Head (ns)	Delta	Status
codedb_bundle	618532	630075	+1.87%	OK
codedb_changes	112411	113102	+0.61%	OK
codedb_deps	31725	30907	-2.58%	OK
codedb_edit	22982	23606	+2.72%	OK
codedb_hot	149075	149938	+0.58%	OK
codedb_outline	397320	384954	-3.11%	OK
codedb_read	169194	169953	+0.45%	OK
codedb_search	250228	246198	-1.61%	OK
codedb_snapshot	3178747	3174188	-0.14%	OK
codedb_status	29353	31549	+7.48%	OK
codedb_symbol	62307	60167	-3.43%	OK
codedb_tree	99828	93853	-5.99%	OK
codedb_word	92572	92813	+0.26%	OK