feat(pi-extension): configurable status-line format

[Smith42]

Refines the status-bar configurability asked for in #324 — the hideStatus half already landed on main but is unreleased; this adds the format half so the line can be restyled without disabling ponytail.

Problem

syncStatus() hardcodes the indicator string ○ 🐴 ponytail: ⚡ FULL. Users who want the always-on ruleset but a different footprint (drop the mascot, shorten ponytail:, compact for narrow footers, drop the label) have to fork the extension or set hideStatus and lose the indicator entirely.

Proposal

One template-style knob that mirrors the existing hideStatus / quietStartup resolution pattern:

• env PONYTAIL_STATUS_FORMAT, else
• statusFormat field in ~/.config/ponytail/config.json, else
• built-in default '{indicator} {emoji} {label} {modeIcon} {mode}'

Placeholders (replaced with String.replaceAll; unknown ones pass through literally so a typo renders rather than vanishes):

Placeholder	Replaced with	Example
{indicator}	○ idle, ● accent when a turn runs	○
{emoji}	the 🐴 mascot	🐴
{label}	ponytail: (muted)	ponytail:
{modeIcon}	🌿 lite, ⚡ full, 🔥 ultra	🔥
{mode}	level uppercased (text color)	ULTRA

The default template reproduces the current line byte-for-byte, so existing setups are unchanged. hideStatus still wins when both are set.

Examples

{ "statusFormat": "{indicator} PT {mode}" }

PONYTAIL_STATUS_FORMAT='{indicator} {mode} 🐴' pi …

Changes

• hooks/ponytail-config.js: getStatusFormat() (BOM-stripped read, same env/config precedence as getHideStatus)
• pi-extension/index.js: syncStatus() renders the template; DEFAULT_STATUS_FORMAT constant co-located for testability
• pi-extension/test/extension.test.js: baseline-equality, env-var, config-file, unknown-placeholder pass-through tests
• README.md: placeholder table + examples under a new "Status-line format (pi)" subsection

Tests

node --test pi-extension/test/extension.test.js → 15/15 pass. The tests/correctness.test.js "csv: correct pandas one-liner passes" failure is pre-existing on clean main (it exercises an external AI judge / pandas) and independent of this change.

Notes

• Backward compatible: no config file → identical output.
• review mode is session-only and never a runtime default (already enforced on main), so no new template exposure for it.
• Surface area is one new config key + one env var; no new abstractions, following the repo's lazy-minimal config style.