docs: add source installation guide

[Kaguya-19]

Summary

• add a dedicated source installation guide with platform-specific dependency commands
• keep the main README concise by linking to the detailed guide
• cover Node.js/node:sqlite, Git/Git LFS, native build tools, ripgrep, clone/install, onboarding, startup, and troubleshooting

Context

• follow-up to docs: clarify source deployment bootstrap #374; docs: clarify source deployment bootstrap #374 was merged before detailed source-install dependency guidance was added

Verification

• documentation-only change
• checked requirements against install.sh, Dockerfile, and package scripts

[Kaguya-19]

Updated verification and China network guidance: I tested the source install guide from a clean clone with GIT_LFS_SKIP_SMUDGE=1 through npm install, UI install, builds, bootstrap, and a short production startup readiness check. I also added Chinese docs for npm registry mirror usage and fnm Node.js mirror usage, and verified the npm registry commands with an isolated npm config.

[Kaguya-19]

Brand-new install verification: I used an isolated temp HOME, PILOT_HOME, and npm cache. Remote GitHub clone of the PR branch hit in the current network, so I documented that failure mode in the Chinese guide. Using a local clone to simulate a fresh checkout, the documented install path completed: root npm install, UI npm install, bootstrap with empty PILOT_HOME, root/UI builds, and short production startup until both UI and Gateway were ready.

[Kaguya-19]

Correction to the previous verification comment: the remote GitHub clone failure was fetch-pack: unexpected disconnect. I added this exact failure mode to the Chinese source install guide, along with retry/proxy guidance and the GIT_LFS_SKIP_SMUDGE=1 workaround for skipping large media assets.