chore: release v0.1.0-rc3

[ycpss91255]

Summary

Release candidate v0.1.0-rc3: rolls up the 0.1.0 TUI redesign (58 commits since
v0.1.0-rc2) on a green, architecture-complete main.

Included:

• fzf two-pane navigator Rich tier + whiptail Fallback tier feature parity (ADR-0024)
• TUI delegates text input to the CLI (ADR-0025)
• apt-essentials split into per-tool base modules (ADR-0026)
• Sidecar at the phase-invocation layer (ADR-0027)
• State / Environment / module-bootstrap / archetype deepenings (refactor(engine): merge detect + platform into one deep Environment module (deepening #5) #253-refactor(module): Sidecar to phase-invocation layer + archetypes emit all 10 (deepening #2+#4) #256)
• screen registry + CLI-fork data broker (refactor(tui): screen registry + CLI-fork data broker (deepening #6 + #7) #257)
• gum removed as a TUI backend (gum stays an installable tool)

Bumps .version to v0.1.0-rc3 and promotes the accumulated [Unreleased]
CHANGELOG section. No breaking changes from v0.1.0-rc2 (pre-1.0 redesign milestone).

Test plan

• CI green on every PR rolled into this release (docs(tui): ADR 0024-0026 + CONTEXT glossary for 0.1.0 TUI redesign #243-refactor(tui): drop gum as a TUI backend (ADR-0024) #259)
• main CI green on the merge of refactor(tui): drop gum as a TUI backend (ADR-0024) #259 (42af1d4: CI = success)
• After merge: tag v0.1.0-rc3 via release-tag.sh and verify CI re-runs cleanly on the tag