docs: reframe stale citations of removed bitty-runtime palette.rs

[Xuepoo]

Priority: P2 | Area: area:plugins | Labels: docs, P2, area:plugins | Milestone: v0.1.0 | RFC: OQ-053 | Task: CTX-0008

Closes #28

Summary

`bitty` PR #1611 (commit `67cdcc14`) removed `crates/bitty-runtime/src/palette.rs`, and the palette manifest had already left `crates/bitty-plugin-host/src/bundled.rs` under CTX-0397. This PR rewords every reference that still treated the bundled Rust realization as live:

• `lua/palette/filter.lua` header: this module is the authoritative realization of the envelope; the bounds came from the historical bundled realization, which bitty PR #1611 removed.
• `README.md`: Behavior intro, plus the "Capability difference from the former bundled realization" section, which now records the removal points. The dead "reconcile the bundled manifest" follow-up is dropped.
• `lua/palette/init.lua`, `bitty-plugin.toml`, `tests/spec/filter_spec.lua` comments, and `AGENTS.md`: past-tense / "former" framing.
• `CHANGELOG.md`: Unreleased/Changed entry.

The numeric bounds (128 entries/query/text, `MAX_OVERLAY_TEXT_LEN`, `MAX_INPUT` 1024) are unchanged. Comments and prose only; no executable change.

Tested

• `just check` (markdownlint, prettier, bitty-plugin-lint, luaparse, Lua 5.4 suite 83/83, LuaLS conformance): green
• `actionlint`, `gitleaks detect --source .`: clean

The PR touches `.lua`/`.toml` comments, so it does not qualify for the docs-only fast path. It waits for the full CI gate.