feat(git): p10k symbols, two-tier defaults, branch glyph and divergence

[jals1212]

Summary

Restyles the native git module (stacked on #15 — merges after it):

• Powerlevel10k-minimal output: main ✓ clean; main ⇡2 ⇣1 ✖1 +2 !3 −1 ?2 diverged and dirty — branch glyph concatenated onto the name (E0A0's own right bearing is the gap), symbol-before-count groups in fixed order (conflicts, staged, modified, deleted, untracked), zero groups omitted. The old global dirty marker is gone: the counts say dirty, the accent escalation says it in colour.
• Upstream divergence: ⇡N/⇣N parsed from the porcelain -b header, shown only when nonzero. Divergence never triggers the dirty escalation (work-tree only) — a diverged-but-clean tree keeps the resting accent. No remote name, no stash — kept simple by design.
• Two-tier symbol defaults: every symbol (branch, clean, conflict, staged, modified, deleted, untracked, ahead, behind) has a glyph default and a plain-ASCII default ((empty) ok x + ! - ? ^ v), selected through the leading_show cascade — label/off means "no icons wanted" and degrades the whole set. An explicit @themux_git_symbol_* always wins.
• Module icon default is the git logo 󰊢.

Changes

File	Change
modules/git.conf	Icon, nine two-tier symbol defaults, @_tmx_git_plain flag from the leading_show cascade, 10-arg script invocation
utils/git_status.sh	Symbol-first formatting, divergence header parse, positional symbol args (${2-} honours the designed-empty branch symbol)
tests/git_module.sh (+expected)	Both tiers, divergence fixtures (local upstream, no network, identity-safe), byte-exact branch-symbol assertion, clean-but-diverged --dirty=0
docs/reference/status-line.md	Output shape, symbol table with both tiers, divergence and escalation notes

Test plan

• Full suite green: run_tests.sh — 17/17
• shellcheck -a clean
• Fresh adversarial review: no CRITICAL findings; header parsing verified safe against adversarial branch names (refnames cannot contain space or [); conflict/divergence fixtures deterministic and CI-identity-safe
• Glyph bytes verified exactly (icon f3 b0 8a a2, branch ee 82 a0, deleted U+2212)

Notes

• The @themux_all_leading_show tier is documented as only reachable when the module tier is unset (stock options bake icon at the module tier) — pre-existing cascade behaviour shared with the metric modules, called out in the conf comment; a repo-wide fix would follow the @themux_all_<prop> read-time pattern in a separate change.