Add CompletionDirection (forward/backward) to completion pipeline

[curtisman]

Summary

Replace CommitMode ("eager"/"explicit") with a host-provided CompletionDirection ("forward"/"backward") signal threaded through the entire completion pipeline. This lets the shell tell the backend whether the user is moving ahead (typing, selecting) or reconsidering (backspacing), eliminating the grammar-side heuristic that tried to guess user intent.

Changes by layer

Grammar matcher (actionGrammar/grammarMatcher.ts)

• Support backward completion for literal string parts and varNumber parts via emitBackwardCompletion()
• Track lastMatchedPartInfo (string or number) on MatchState for backing up to the most recently matched item
• Refactor duplicate emit logic into three helpers: emitPropertyCompletion, emitStringCompletion, emitBackwardCompletion
• Compute directionSensitive from couldBackUp (matched parts without trailing separator) in tryPartialStringMatch
• Add consumeTrailingSeparators() so trailing whitespace is included in matchedPrefixLength
• New required field directionSensitive on GrammarCompletionResult

Construction cache (cache/constructionCache.ts)

• Thread direction through completion() and related methods
• Use matchedStarts for backward to locate previous part position
• Track noTrailingSeparator × partialPartCount ≥ 1 for directionSensitive

SDK & RPC

• Add CompletionDirection = "forward" | "backward" type to agentSdk
• Thread direction through agentRpc client/server and dispatcher RPC types

Dispatcher (dispatcher/completion.ts)

• Replace CommitMode with directionSensitive: boolean on CommandCompletionResult
• Replace tokenBoundary() (whitespace rewind) with hasWhitespaceBefore() — trailing whitespace is now consumed as a commit signal rather than stripped
• Rename uncommittedCommand → reconsideringCommand — direction-gated instead of trailing-space-gated
• Thread separatorMode through CompletionTarget; agent only overrides when it advances startIndex via matchedPrefixLength
• OR-merge directionSensitive across target + agent results

Shell (partialCompletionSession.ts)

• Auto-detect direction: backward only when new input is a strict prefix of previous input (genuine backspace)
• Store directionSensitive and lastDirection from backend result
• New re-fetch trigger A7: direction changed AND directionSensitive AND input === anchor
• Once the user types past the anchor, loaded completions are reused regardless of direction change

Documentation

• Update docs/architecture/completion.md with direction model, category table showing backward behavior, and directionSensitive field
• Add breadcrumb comments (Architecture: docs/architecture/completion.md — §N) in all source files

Tests

Grammar (actionGrammar)

• 40 directionSensitive flag tests: single string, multi-part, wildcard, Category 3b, varNumber, optional ()?, repeat ()+/()*, spacing=none backward, minPrefixLength + backward, forward/backward agreement
• Prefix length tests for optional backward completions

Construction cache (cache)

• 8 backward direction tests
• 5 conflicting directionSensitive tests: sensitive-wins, any-sensitive, longer-match-resets, trailing-space-commits

Dispatcher

• 7 directionSensitive flag tests (exact subcommand, trailing space, pending flag, boolean flag, empty input)
• 8 backward direction tests (subcommand reconsideration, nested commands, flag backtrack, empty input)
• 3 flat agent backward tests (no subcommand table, boolean flag, trailing space)
• 1 graceful fallback test (unknown agent with backward)
• 2 catch-block tests with a throwing agent (throwtest) that exercises the error recovery path

Shell

• 5 direction detection pattern tests (backspace, different text, forward typing)
• 3 direction-change-while-PENDING tests (re-fetch trigger A7)
• 2 spacePunctuation + direction interaction tests
• All existing tests updated for parameter threading