fix(plugin): codexRemove drops header-only bili block with no trailing newline

[ranxianglei]

Bug

codexRemove() in src/plugin-install.ts leaves the [mcp_servers.bili] header line behind when the table is header-only (no keys) and sits as the final line with no trailing newline.

Root cause:

const nextTable = after.slice(after.indexOf("\n") + 1).search(/^[ \t]*\[/m);
const end = nextTable >= 0 ? start + after.indexOf("\n") + 1 + nextTable : text.length;

When after.indexOf("\n") is -1 (no newline in the block), after.slice(0) still includes the header itself, so the next-table search matches the header at index 0. end collapses to start and the header line survives the removal.

Reproduced: a config ending in ...[mcp_servers.bili] (no trailing \n) still contains mcp_servers.bili after pluginRemove("codex").

Fix

Compute firstNewline once and treat a missing newline as "block runs to EOF":

const firstNewline = after.indexOf("\n");
const nextTable = firstNewline < 0 ? -1 : after.slice(firstNewline + 1).search(/^[ \t]*\[/m);
const end = nextTable >= 0 ? start + firstNewline + 1 + nextTable : text.length;

Notes

• Very low real-world likelihood: codexInstall always writes command/args/env keys, so a header-only block only arises from manual editing, and it additionally requires a missing trailing newline. Fixing it anyway since the removal path should be total.
• Adds a regression test in tests/plugin-agent.test.ts covering the header-only, no-trailing-newline case. Verified the test fails on the old code (header survives) and passes with the fix.
• Full suite: 512 pass / 0 fail.