perf(build): stop full-tree recompiles triggered by git activity

The jcode-build-meta build script embeds version/git metadata that every
crate in the graph (base -> app-core -> tui -> cli) consumes via
`env!("JCODE_*")`. Two design issues made routine git activity force a
full-tree recompile (~18-25s) on essentially every incremental build:

1. It declared `.git/HEAD` and `.git/index` as `rerun-if-changed` inputs.
   Those mtimes change on every `git add`, `git status`, commit, and
   concurrent-agent git op. Cargo treats a build script as dirty when any
   declared input is newer than its output, reruns it, and then force-
   recompiles all dependents via StaleDepFingerprint -- even when the
   emitted output is byte-identical.

2. It auto-incremented a persistent patch counter on every rerun, so the
   embedded JCODE_SEMVER/JCODE_VERSION genuinely changed each time,
   guaranteeing the cascade above actually invalidated everything.

Fixes:
- Derive the dev patch number deterministically from committed git state
  (`base.patch + commits-since-base-tag`) instead of a side-effecting
  counter. Pure function of HEAD; stable across rebuilds. Removes ~150
  lines of counter + file-locking machinery.
- Drop the `.git/HEAD`/`.git/index` rerun triggers. Keep Cargo.toml and
  the JCODE_RELEASE_BUILD / JCODE_BUILD_SEMVER / JCODE_BUILD_GIT_* env
  triggers so release/dist builds (which set those) still embed exact
  metadata, and a version bump still refreshes it.

Measured on this machine (selfdev, warm):
  git/index touch:  ~18-25s  ->  0.65s   (~30x)
  git/HEAD  touch:  ~18s     ->  0.87s
  real code edit:   ~20s     ->  ~20s    (correctly unchanged)

Dev `--version` may lag the latest in-session commit's hash until the next
real rebuild; that is cosmetic and an acceptable trade.